Extract compass support into extension, rewrite all features that rely on it to be pure Ruby

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

@@ -14,6 +14,12 @@ module Middleman
         app.define_hook :build_config
         app.define_hook :development_config
 
+        app.config.define_setting :autoload_sprockets, true, 'Automatically load sprockets at startup?'
+        app.config[:autoload_sprockets] = (ENV['AUTOLOAD_SPROCKETS'] == 'true') if ENV['AUTOLOAD_SPROCKETS']
+
+        app.config.define_setting :autoload_compass, true, 'Automatically load compass at startup?'
+        app.config[:autoload_compass] = (ENV['AUTOLOAD_COMPASS'] == 'true') if ENV['AUTOLOAD_COMPASS']
+
         app.extend ClassMethods
         app.delegate :configure, to: :"self.class"
       end
@@ -90,7 +96,7 @@ module Middleman
           activate ext_name
         end
 
-        if ENV['AUTOLOAD_SPROCKETS'] != 'false'
+        if config[:autoload_sprockets]
           begin
             require 'middleman-sprockets'
             activate :sprockets
@@ -99,6 +105,15 @@ module Middleman
           end
         end
 
+        if config[:autoload_compass]
+          begin
+            require 'middleman-compass'
+            activate :compass
+          rescue LoadError
+            # Compass is not available, don't complain about it
+          end
+        end
+
         # Evaluate a passed block if given
         config_context.instance_exec(&block) if block_given?