Simplify configuration a bit, and make extensions raise an error if the user tries to set an invalid extension option.

middleman-core/lib/middleman-core/configuration.rb

@@ -165,16 +165,12 @@ module Middleman
       # mode and no new settings may be defined.
       def finalize!
         @finalized = true
+        self
       end
 
       # Deep duplicate of the configuration manager
       def dup
-        copy = ConfigurationManager.new
-        @settings.each do |key, setting|
-          copy_setting = copy.define_setting(setting.key, setting.default, setting.description)
-          copy_setting.value = setting.value if setting.value_set?
-        end
-        copy
+        ConfigurationManager.new.tap {|c| c.load_settings(self.all_settings) }
       end
 
       # Load in a list of settings

middleman-core/lib/middleman-core/extensions.rb

@@ -114,15 +114,15 @@ module Middleman
 
     attr_accessor :app, :options
 
-    def initialize(klass, options_hash={}, &block)
-      @options = ::Middleman::Configuration::ConfigurationManager.new
-      @options.load_settings(self.class.config.all_settings)
+    def initialize(klass, options_hash={})
+      @options = self.class.config.dup
+      @options.finalize!
 
       options_hash.each do |k, v|
         @options[k] = v
       end
 
-      block.call(@options) if block_given?
+      yield @options if block_given?
 
       ext = self
       klass.after_configuration do
@@ -132,7 +132,6 @@ module Middleman
     end
 
     def after_configuration
-
       nil
     end
   end