diff --git a/.yardopts b/.yardopts
index 980f0e4c..8a5eba70 100644
--- a/.yardopts
+++ b/.yardopts
@@ -7,4 +7,5 @@ middleman-*/lib/**/*.rb
 --exclude middleman-cli/lib/middleman-cli/templates/shared/
 --exclude middleman-cli/lib/middleman-cli/templates/extension/
 --no-private
---hide-void-return
\ No newline at end of file
+--hide-void-return
+--markup=markdown
\ No newline at end of file
diff --git a/middleman-cli/.yardopts b/middleman-cli/.yardopts
index 47ae041b..b816052e 100644
--- a/middleman-cli/.yardopts
+++ b/middleman-cli/.yardopts
@@ -5,4 +5,5 @@ lib/**/*.rb
 --exclude lib/middleman-cli/templates/shared/
 --exclude lib/middleman-cli/templates/extension/
 --no-private
---hide-void-return
\ No newline at end of file
+--hide-void-return
+--markup=markdown
\ No newline at end of file
diff --git a/middleman-core/.yardopts b/middleman-core/.yardopts
index 29455a55..ee16b7ac 100644
--- a/middleman-core/.yardopts
+++ b/middleman-core/.yardopts
@@ -2,4 +2,5 @@ lib/**/*.rb
 --exclude lib/vendored-middleman-deps/
 --exclude lib/middleman-core/step_definitions
 --no-private
---hide-void-return
\ No newline at end of file
+--hide-void-return
+--markup=markdown
\ No newline at end of file
diff --git a/middleman-core/lib/middleman-core/auto_gem_extensions.rb b/middleman-core/lib/middleman-core/auto_gem_extensions.rb
new file mode 100644
index 00000000..430106b2
--- /dev/null
+++ b/middleman-core/lib/middleman-core/auto_gem_extensions.rb
@@ -0,0 +1,55 @@
+# Add module methods to the Middleman module that allow automatically loading
+# extensions defined in gems based on the existance of a special file. This also
+# adds a method for iterating over all the Gems on a system.
+module Middleman
+  class << self
+    # Where to look in gems for extensions to auto-register. Since most extensions are
+    # called out in a Gemfile, this is really only useful for template extensions that get
+    # used by "middleman init".
+    EXTENSION_FILE = File.join('lib', 'middleman_extension.rb') unless const_defined?(:EXTENSION_FILE)
+
+    # Automatically load extensions from available RubyGems
+    # which contain the EXTENSION_FILE
+    #
+    # @private
+    def load_extensions_in_path
+      require 'rubygems'
+
+      extensions = rubygems_latest_specs.select do |spec|
+        spec_has_file?(spec, EXTENSION_FILE)
+      end
+
+      extensions.each do |spec|
+        require spec.name
+      end
+    end
+
+    # Backwards compatible means of finding all the latest gemspecs
+    # available on the system
+    #
+    # @private
+    # @return [Array] Array of latest Gem::Specification
+    def rubygems_latest_specs
+      # If newer Rubygems
+      if ::Gem::Specification.respond_to? :latest_specs
+        ::Gem::Specification.latest_specs(true)
+      else
+        ::Gem.source_index.latest_specs
+      end
+    end
+
+    private
+
+    # Where a given Gem::Specification has a specific file. Used
+    # to discover extensions.
+    #
+    # @private
+    # @param [Gem::Specification] spec
+    # @param [String] path Path to look for
+    # @return [Boolean] Whether the file exists
+    def spec_has_file?(spec, path)
+      full_path = File.join(spec.full_gem_path, path)
+      File.exist?(full_path)
+    end
+  end
+end
diff --git a/middleman-core/lib/middleman-core/configuration.rb b/middleman-core/lib/middleman-core/configuration.rb
index dce449e8..7c39a405 100644
--- a/middleman-core/lib/middleman-core/configuration.rb
+++ b/middleman-core/lib/middleman-core/configuration.rb
@@ -82,9 +82,11 @@ module Middleman
       # Define a new setting, with optional default and user-friendly description.
       # Once the configuration manager is finalized, no new settings may be defined.
       #
-      # @param [Symbol] key
-      # @param [Object] default
-      # @param [String] description
+      # @example
+      #   config.define_setting :compress, false, 'Whether to compress the output'
+      # @param [Symbol] key The name of the option
+      # @param [Object] default The default value for the option
+      # @param [String] description A human-readable description of what the option does
       # @return [ConfigSetting]
       def define_setting(key, default=nil, description=nil)
         raise "Setting #{key} doesn't exist" if @finalized
diff --git a/middleman-core/lib/middleman-core/core_extensions/extensions.rb b/middleman-core/lib/middleman-core/core_extensions/extensions.rb
index b48175c4..8aad51f9 100644
--- a/middleman-core/lib/middleman-core/core_extensions/extensions.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/extensions.rb
@@ -1,34 +1,3 @@
-# Middleman provides an extension API which allows you to hook into the
-# lifecycle of a page request, or static build, and manipulate the output.
-# Internal to Middleman, these extensions are called "features," but we use
-# the exact same API as is made available to the public.
-#
-# A Middleman extension looks like this:
-#
-#     module MyExtension
-#       class << self
-#         def registered(app)
-#           # My Code
-#         end
-#       end
-#     end
-#
-# In your `config.rb`, you must load your extension (if it is not defined in
-# that file) and call `activate`.
-#
-#     require "my_extension"
-#     activate MyExtension
-#
-# This will call the `registered` method in your extension and provide you
-# with the `app` parameter which is a Middleman::Application context. From here
-# you can choose to respond to requests for certain paths or simply attach
-# Rack middleware to the stack.
-#
-# The built-in features cover a wide range of functions. Some provide helper
-# methods to use in your views. Some modify the output on-the-fly. And some
-# apply computationally-intensive changes to your final build files.
-
-# Namespace extensions module
 module Middleman
   module CoreExtensions
     module Extensions
@@ -47,7 +16,6 @@ module Middleman
           app.config[:autoload_sprockets] = (ENV['AUTOLOAD_SPROCKETS'] == 'true') if ENV['AUTOLOAD_SPROCKETS']
 
           app.extend ClassMethods
-          app.send :include, InstanceMethods
           app.delegate :configure, to: :"self.class"
         end
       end
@@ -56,121 +24,119 @@ module Middleman
       module ClassMethods
         # Add a callback to run in a specific environment
         #
-        # @param [String, Symbol] env The environment to run in
+        # @param [String, Symbol] env The environment to run in (:build, :development)
         # @return [void]
         def configure(env, &block)
           send("#{env}_config", &block)
         end
       end
 
-      # Instance methods
-      module InstanceMethods
-        # This method is available in the project's `config.rb`.
-        # It takes a underscore-separated symbol, finds the appropriate
-        # feature module and includes it.
-        #
-        #     activate :lorem
-        #
-        # @param [Symbol, Module] ext Which extension to activate
-        # @return [void]
-        # rubocop:disable BlockNesting
-        def activate(ext, options={}, &block)
-          extension = ::Middleman::Extensions.load(ext)
-          logger.debug "== Activating: #{ext}"
+      # This method is available in the project's `config.rb`.
+      # It takes a underscore-separated symbol, finds the appropriate
+      # feature module and includes it.
+      #
+      #     activate :lorem
+      #
+      # @param [Symbol, Module] ext Which extension to activate
+      # @return [void]
+      # rubocop:disable BlockNesting
+      def activate(ext, options={}, &block)
+        extension = ::Middleman::Extensions.load(ext)
+        logger.debug "== Activating: #{ext}"
 
-          if extension.supports_multiple_instances?
-            extensions[ext] ||= {}
-            key = "instance_#{extensions[ext].keys.length}"
-            extensions[ext][key] = extension.new(self.class, options, &block)
+        if extension.supports_multiple_instances?
+          extensions[ext] ||= {}
+          key = "instance_#{extensions[ext].keys.length}"
+          extensions[ext][key] = extension.new(self.class, options, &block)
+        else
+          if extensions[ext]
+            raise "#{ext} has already been activated and cannot be re-activated."
           else
-            if extensions[ext]
-              raise "#{ext} has already been activated and cannot be re-activated."
-            else
-              extensions[ext] = extension.new(self.class, options, &block)
-            end
+            extensions[ext] = extension.new(self.class, options, &block)
+          end
+        end
+      end
+
+      # Access activated extensions
+      #
+      # @return [Hash<Symbol,Middleman::Extension|Module>]
+      def extensions
+        @extensions ||= {}
+      end
+
+      # Load features before starting server
+      def initialize
+        super
+
+        self.class.inst = self
+
+        # Search the root of the project for required files
+        $LOAD_PATH.unshift(root)
+
+        ::Middleman::Extension.clear_after_extension_callbacks
+
+        if config[:autoload_sprockets]
+          begin
+            require 'middleman-sprockets'
+            activate(:sprockets)
+          rescue LoadError
           end
         end
 
-        # Access activated extensions
-        #
-        # @return [Hash<Symbol,Middleman::Extension|Module>]
-        def extensions
-          @extensions ||= {}
+        run_hook :initialized
+
+        run_hook :before_configuration
+
+        # Check for and evaluate local configuration
+        local_config = File.join(root, 'config.rb')
+        if File.exist? local_config
+          logger.debug '== Reading:  Local config'
+          config_context.instance_eval File.read(local_config), local_config, 1
         end
 
-        # Load features before starting server
-        def initialize
-          super
+        if build?
+          run_hook :build_config
+          config_context.execute_configure_callbacks(:build)
+        end
 
-          self.class.inst = self
+        if development?
+          run_hook :development_config
+          config_context.execute_configure_callbacks(:development)
+        end
 
-          # Search the root of the project for required files
-          $LOAD_PATH.unshift(root)
+        run_hook :instance_available
 
-          ::Middleman::Extension.clear_after_extension_callbacks
+        # This is for making the tests work - since the tests
+        # don't completely reload middleman, I18n.load_path can get
+        # polluted with paths from other test app directories that don't
+        # exist anymore.
+        if ENV['TEST']
+          ::I18n.load_path.delete_if { |path| path =~ %r{tmp/aruba} }
+          ::I18n.reload!
+        end
 
-          if config[:autoload_sprockets]
-            begin
-              require 'middleman-sprockets'
-              activate(:sprockets)
-            rescue LoadError
+        run_hook :after_configuration
+        config_context.execute_after_configuration_callbacks
+
+        logger.debug 'Loaded extensions:'
+        extensions.each do |ext, klass|
+          if ext.is_a?(Hash)
+            ext.each do |k, _|
+              logger.debug "== Extension: #{k}"
             end
+          else
+            logger.debug "== Extension: #{ext}"
           end
 
-          run_hook :initialized
-
-          run_hook :before_configuration
-
-          # Check for and evaluate local configuration
-          local_config = File.join(root, 'config.rb')
-          if File.exist? local_config
-            logger.debug '== Reading:  Local config'
-            config_context.instance_eval File.read(local_config), local_config, 1
-          end
-
-          if build?
-            run_hook :build_config
-            config_context.execute_configure_callbacks(:build)
-          end
-
-          if development?
-            run_hook :development_config
-            config_context.execute_configure_callbacks(:development)
-          end
-
-          run_hook :instance_available
-
-          # This is for making the tests work - since the tests
-          # don't completely reload middleman, I18n.load_path can get
-          # polluted with paths from other test app directories that don't
-          # exist anymore.
-          if ENV['TEST']
-            ::I18n.load_path.delete_if { |path| path =~ %r{tmp/aruba} }
-            ::I18n.reload!
-          end
-
-          run_hook :after_configuration
-          config_context.execute_after_configuration_callbacks
-
-          logger.debug 'Loaded extensions:'
-          extensions.each do |ext, klass|
-            if ext.is_a?(Hash)
-              ext.each do |k, _|
-                logger.debug "== Extension: #{k}"
-              end
-            else
-              logger.debug "== Extension: #{ext}"
+          if klass.is_a?(::Middleman::Extension)
+            # Forward Extension helpers to TemplateContext
+            (klass.class.defined_helpers || []).each do |m|
+              @template_context_class.send(:include, m)
             end
 
-            if klass.is_a?(::Middleman::Extension)
-              # Forward Extension helpers to TemplateContext
-              (klass.class.defined_helpers || []).each do |m|
-                @template_context_class.send(:include, m)
-              end
-
-              ::Middleman::Extension.activated_extension(klass)
-            end
+            ::Middleman::Extension.activated_extension(klass)
           end
+
         end
       end
     end
diff --git a/middleman-core/lib/middleman-core/core_extensions/request.rb b/middleman-core/lib/middleman-core/core_extensions/request.rb
index c8155198..3e822715 100644
--- a/middleman-core/lib/middleman-core/core_extensions/request.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/request.rb
@@ -56,8 +56,6 @@ module Middleman
         # Set the shared instance
         #
         # @private
-        # @param [Middleman::Application] inst
-        # @return [void]
         attr_writer :inst
 
         # Return built Rack app
@@ -188,7 +186,6 @@ module Middleman
         # message.
         #
         # @param env
-        # @param [Rack::Request] req
         # @param [Rack::Response] res
         def process_request(env, _, res)
           start_time = Time.now
diff --git a/middleman-core/lib/middleman-core/extension.rb b/middleman-core/lib/middleman-core/extension.rb
index 80865c3a..f4248657 100644
--- a/middleman-core/lib/middleman-core/extension.rb
+++ b/middleman-core/lib/middleman-core/extension.rb
@@ -2,64 +2,179 @@ require 'active_support/core_ext/module/delegation'
 require 'active_support/core_ext/class/attribute'
 
 module Middleman
+  # Middleman's Extension API provides the ability to add functionality to Middleman
+  # and to customize existing features. Internally, most features in Middleman are
+  # implemented as extensions. A good way to figure out how to write your own extension
+  # is to look at the source of the built-in extensions or popular extension gems like
+  # `middleman-blog` or `middleman-syntax`.
+  #
+  # The most basic extension looks like:
+  #
+  #     class MyFeature < Middleman::Extension
+  #       def initialize(app, options_hash={}, &block)
+  #         super
+  #       end
+  #     end
+  #     ::Middleman::Extensions.register(:my_feature, MyFeature)
+  #
+  # A more complicated example might look like:
+  #
+  #     class MyFeature < Middleman::Extension
+  #       option :my_option, 'cool', 'A very cool option'
+  #
+  #       def initialize(app, options_hash={}, &block)
+  #         super
+  #         puts "My option is #{options.my_option}"
+  #       end
+  #
+  #       def after_configuration
+  #         puts "The project has been configured"
+  #       end
+  #
+  #       def manipulate_resource_list(resources)
+  #         resources.each do |resource|
+  #           # Make all .jpg's get built or served with a .jpeg extension.
+  #           if resource.ext == '.jpg'
+  #             resource.destination_path = resource.destination_path.sub('.jpg', '.jpeg')
+  #           end
+  #         end
+  #       end
+  #     end
+  #
+  #     ::Middleman::Extensions.register :my_feature do
+  #       MyFeature
+  #     end
+  #
+  # Extensions can add helpers (via {Extension.helpers}), add to the sitemap or change it (via {#manipulate_resource_list}), or run
+  # arbitrary code at different parts of the Middleman application's lifecycle. They can have options (defined via {Extension.option} and accessed via {#options}).
+  #
+  # Common lifecycle events can be handled by extensions simply by implementing an appropriately-named method:
+  #
+  # * {#after_configuration}
+  # * {#after_build}
+  # * {#before_build}
+  # * {#instance_available}
+  #
+  # There are also some less common hooks that can be listened to from within an extension's `initialize` method:
+  #
+  # * `app.before_render {|body, path, locs, template_class| ... }` - Manipulate template sources before they are rendered.
+  # * `app.after_render {|content, path, locs, template_class| ... }` - Manipulate output text after a template has been rendered. It is also common to install a Rack middleware to do this instead.
+  # * `app.ready { ... }` - Run code once Middleman is ready to serve or build files (after `after_configuration`).
+  # * `app.compass_config { |compass_config| ... }` - Manipulate the Compass configuration after it has been set up.
+  #
+  # @see http://middlemanapp.com/advanced/custom/ Middleman Custom Extensions Documentation
   class Extension
+    # @!attribute supports_multiple_instances
+    #   @!scope class
+    #   @return [Boolean] whether or not an extension can be activated multiple times, generating multiple instances of the extension.
+    #   By default extensions can only be activated once in a project. This is an advanced option.
     class_attribute :supports_multiple_instances, instance_reader: false, instance_writer: false
+
+    # @!attribute defined_helpers
+    #   @!scope class
+    #   @api private
+    #   @return [Array<Module>] a list of all the helper modules this extension provides. Set these using {#helpers}.
     class_attribute :defined_helpers, instance_reader: false, instance_writer: false
+
+    # @!attribute ext_name
+    #   @!scope class
+    #   @return [Symbol] the name this extension is registered under. This is the symbol used to activate the extension.
     class_attribute :ext_name, instance_reader: false, instance_writer: false
 
     class << self
+      # @api private
+      # @return [Middleman::Configuration::ConfigurationManager] The defined options for this extension.
       def config
         @_config ||= ::Middleman::Configuration::ConfigurationManager.new
       end
 
+      # Add an option to this extension.
+      # @see Middleman::Configuration::ConfigurationManager#define_setting
+      # @example
+      #   option :compress, false, 'Whether to compress the output'
+      # @param [Symbol] key The name of the option
+      # @param [Object] default The default value for the option
+      # @param [String] description A human-readable description of what the option does
       def option(key, default=nil, description=nil)
         config.define_setting(key, default, description)
       end
 
-      # Add helpers to the global Middleman application.
+      # Declare helpers to be added the global Middleman application.
       # This accepts either a list of modules to add on behalf
       # of this extension, or a block whose contents will all
       # be used as helpers in a new module.
-      def helpers(*m, &block)
+      # @example With a block:
+      #   helpers do
+      #     def my_helper
+      #       "I helped!"
+      #     end
+      #   end
+      # @example With modules:
+      #   helpers FancyHelpers, PlainHelpers
+      # @param [Array<Module>] modules An optional list of modules to add as helpers
+      # @param [Proc] block A block which will be evaluated to create a new helper module
+      # @return [void]
+      def helpers(*modules, &block)
         self.defined_helpers ||= []
 
         if block_given?
           mod = Module.new
           mod.module_eval(&block)
-          m = [mod]
+          modules = [mod]
         end
 
-        self.defined_helpers += m
-      end
-
-      def activate
-        new(::Middleman::Application)
+        self.defined_helpers += modules
       end
 
+      # Reset all {Extension.after_extension_activated} callbacks.
+      # @api private
+      # @return [void]
       def clear_after_extension_callbacks
         @_extension_activation_callbacks = {}
       end
 
+      # Register to run a block after a named extension is activated.
+      # @param [Symbol] name The name the extension was registered under
+      # @param [Proc] block A callback to run when the named extension is activated
+      # @return [void]
       def after_extension_activated(name, &block)
         @_extension_activation_callbacks ||= {}
         @_extension_activation_callbacks[name] ||= []
         @_extension_activation_callbacks[name] << block if block_given?
       end
 
+      # Notify that a particular extension has been activated and run all
+      # registered {Extension.after_extension_activated} callbacks.
+      # @api private
+      # @param [Middleman::Extension] instance Activated extension instance
+      # @return [void]
       def activated_extension(instance)
         name = instance.class.ext_name
-        return unless @_extension_activation_callbacks && @_extension_activation_callbacks[name]
+        return unless @_extension_activation_callbacks && @_extension_activation_callbacks.has_key?(name)
         @_extension_activation_callbacks[name].each do |block|
           block.arity == 1 ? block.call(instance) : block.call
         end
       end
     end
 
-    attr_accessor :options
+    # @return [Middleman::Configuration::ConfigurationManager] options for this extension instance.
+    attr_reader :options
+
+    # @return [Middleman::Application] the Middleman application instance.
     attr_reader :app
 
+    # @!method after_extension_activated(name, &block)
+    #   Register to run a block after a named extension is activated.
+    #   @param [Symbol] name The name the extension was registered under
+    #   @param [Proc] block A callback to run when the named extension is activated
+    #   @return [void]
     delegate :after_extension_activated, to: :"::Middleman::Extension"
 
+    # Extensions are instantiated when they are activated.
+    # @param [Class] klass The Middleman::Application class
+    # @param [Hash] options_hash The raw options hash. Subclasses should not manipulate this directly - it will be turned into {#options}.
+    # @yield An optional block that can be used to customize options before the extension is activated.
+    # @yieldparam [Middleman::Configuration::ConfigurationManager] options Extension options
     def initialize(klass, options_hash={}, &block)
       @_helpers = []
       @klass = klass
@@ -74,6 +189,39 @@ module Middleman
       bind_after_build
     end
 
+    # @!method before_configuration
+    #   Respond to the `before_configuration` event.
+    #   If a `before_configuration` method is implemented, that method will be run before `config.rb` is run.
+    #   @note Because most extensions are activated from within `config.rb`, they *will not run* any `before_configuration` hook.
+
+    # @!method after_configuration
+    #   Respond to the `after_configuration` event.
+    #   If an `after_configuration` method is implemented, that method will be run before `config.rb` is run.
+
+    # @!method before_build
+    #   Respond to the `before_build` event.
+    #   If an `before_build` method is implemented, that method will be run before the builder runs.
+
+    # @!method after_build
+    #   Respond to the `after_build` event.
+    #   If an `after_build` method is implemented, that method will be run after the builder runs.
+
+    # @!method instance_available
+    #   Respond to the `instance_available` event.
+    #   If an `instance_available` method is implemented, that method will be run after `config.rb` is run and after environment-specific config blocks have been run, but before any `after_configuration` callbacks.
+
+    # @!method manipulate_resource_list(resources)
+    #   Manipulate the resource list by transforming or adding {Sitemap::Resource}s.
+    #   Sitemap manipulation is a powerful way of interacting with a project, since it can modify each {Sitemap::Resource} or generate new {Sitemap::Resources}. This method is used in a pipeline where each sitemap manipulator is run in turn, with each one being fed the output of the previous manipulator. See the source of built-in Middleman extensions like {Middleman::Extensions::DirectoryIndexes} and {Middleman::Extensions::AssetHash} for examples of how to use this.
+    #   @note This method *must* return the full set of resources, because its return value will be used as the new sitemap.
+    #   @see http://middlemanapp.com/advanced/sitemap/ Sitemap Documentation
+    #   @see Sitemap::Store
+    #   @see Sitemap::Resource
+    #   @param [Array<Sitemap::Resource>] resources A list of all the resources known to the sitemap.
+    #   @return [Array<Sitemap::Resource>] The transformed list of resources.
+
+    # Assign the app instance. Used internally.
+    # @api private
     def app=(app)
       @app = app
 
@@ -85,8 +233,10 @@ module Middleman
       end
     end
 
-    protected
+    private
 
+    # @yield An optional block that can be used to customize options before the extension is activated.
+    # @yieldparam Middleman::Configuration::ConfigurationManager] options Extension options
     def setup_options(options_hash)
       @options = self.class.config.dup
       @options.finalize!
diff --git a/middleman-core/lib/middleman-core/extensions.rb b/middleman-core/lib/middleman-core/extensions.rb
index f775b99e..47157122 100644
--- a/middleman-core/lib/middleman-core/extensions.rb
+++ b/middleman-core/lib/middleman-core/extensions.rb
@@ -1,4 +1,10 @@
+require 'middleman-core/extension'
+
 module Middleman
+  # The Extensions module is used to handle global registration and loading of Middleman Extensions.
+  #
+  # The application-facing extension API (activate, etc) is in Middleman::CoreExtensions::Extensions in
+  # middleman-core/core_extensions/extensions.rb.
   module Extensions
     class << self
       def registered
@@ -66,55 +72,4 @@ module Middleman
       end
     end
   end
-
-  # Where to look in gems for extensions to auto-register. Since most extensions are
-  # called out in a Gemfile, this is really only useful for template extensions that get
-  # used by "middleman init".
-  EXTENSION_FILE = File.join('lib', 'middleman_extension.rb') unless const_defined?(:EXTENSION_FILE)
-
-  class << self
-    # Automatically load extensions from available RubyGems
-    # which contain the EXTENSION_FILE
-    #
-    # @private
-    def load_extensions_in_path
-      require 'rubygems'
-
-      extensions = rubygems_latest_specs.select do |spec|
-        spec_has_file?(spec, EXTENSION_FILE)
-      end
-
-      extensions.each do |spec|
-        require spec.name
-      end
-    end
-
-    # Backwards compatible means of finding all the latest gemspecs
-    # available on the system
-    #
-    # @private
-    # @return [Array] Array of latest Gem::Specification
-    def rubygems_latest_specs
-      # If newer Rubygems
-      if ::Gem::Specification.respond_to? :latest_specs
-        ::Gem::Specification.latest_specs(true)
-      else
-        ::Gem.source_index.latest_specs
-      end
-    end
-
-    # Where a given Gem::Specification has a specific file. Used
-    # to discover extensions.
-    #
-    # @private
-    # @param [Gem::Specification] spec
-    # @param [String] path Path to look for
-    # @return [Boolean] Whether the file exists
-    def spec_has_file?(spec, path)
-      full_path = File.join(spec.full_gem_path, path)
-      File.exist?(full_path)
-    end
-  end
 end
-
-require 'middleman-core/extension'
diff --git a/middleman-core/lib/middleman-core/load_paths.rb b/middleman-core/lib/middleman-core/load_paths.rb
index 68defc69..36c32704 100644
--- a/middleman-core/lib/middleman-core/load_paths.rb
+++ b/middleman-core/lib/middleman-core/load_paths.rb
@@ -1,5 +1,6 @@
 # Core Pathname library used for traversal
 require 'pathname'
+require 'middleman-core/auto_gem_extensions'
 
 module Middleman
   class << self
diff --git a/middleman-core/lib/middleman-core/sitemap/store.rb b/middleman-core/lib/middleman-core/sitemap/store.rb
index a987843c..816d94d5 100644
--- a/middleman-core/lib/middleman-core/sitemap/store.rb
+++ b/middleman-core/lib/middleman-core/sitemap/store.rb
@@ -57,21 +57,20 @@ module Middleman
         app.config_context.class.send :delegate, :sitemap, to: :app
       end
 
-      # Register a klass which can manipulate the main site map list. Best to register
-      # these in a before_configuration or after_configuration hook.
+      # Register an object which can transform the sitemap resource list. Best to register
+      # these in a `before_configuration` or `after_configuration` hook.
       #
       # @param [Symbol] name Name of the manipulator for debugging
-      # @param [Class, Module] inst Abstract namespace which can update the resource list
+      # @param [#manipulate_resource_list] manipulator Resource list manipulator
       # @return [void]
-      def register_resource_list_manipulator(name, inst, *)
-        @resource_list_manipulators << [name, inst]
+      def register_resource_list_manipulator(name, manipulator, *)
+        @resource_list_manipulators << [name, manipulator]
         rebuild_resource_list!(:registered_new)
       end
 
       # Rebuild the list of resources from scratch, using registed manipulators
-      # rubocop:disable UnusedMethodArgument
       # @return [void]
-      def rebuild_resource_list!(reason=nil)
+      def rebuild_resource_list!(_=nil)
         @lock.synchronize do
           @needs_sitemap_rebuild = true
         end
diff --git a/middleman-core/lib/middleman-core/template_context.rb b/middleman-core/lib/middleman-core/template_context.rb
index f2d4a4fb..81d405a8 100644
--- a/middleman-core/lib/middleman-core/template_context.rb
+++ b/middleman-core/lib/middleman-core/template_context.rb
@@ -60,7 +60,6 @@ module Middleman
     # Sinatra/Padrino compatible render method signature referenced by some view
     # helpers. Especially partials.
     #
-    # @param [String, Symbol] engine
     # @param [String, Symbol] data
     # @param [Hash] options
     # @return [String]
diff --git a/middleman-templates/.yardopts b/middleman-templates/.yardopts
index 72efde46..fcefc936 100644
--- a/middleman-templates/.yardopts
+++ b/middleman-templates/.yardopts
@@ -1,4 +1,5 @@
 lib/**/*.rb
 --exclude lib/middleman-templates
 --no-private
---hide-void-return
\ No newline at end of file
+--hide-void-return
+--markup=markdown
\ No newline at end of file