Add an extension pointer for custom importer

Author: Rafael Mendonça França

lib/sass/rails/template.rb

@@ -15,8 +15,8 @@ def evaluate(context, locals, &block)
           :line => line,
           :syntax => syntax,
           :cache_store => cache_store,
-          :importer => SassImporter.new(context, context.pathname.to_s),
-          :load_paths => context.environment.paths.map { |path| SassImporter.new(context, path.to_s) },
+          :importer => importer_class.new(context, context.pathname.to_s),
+          :load_paths => context.environment.paths.map { |path| importer_class.new(context, path.to_s) },
           :sprockets => {
             :context => context,
             :environment => context.environment
@@ -30,6 +30,12 @@ def evaluate(context, locals, &block)
         context.__LINE__ = e.sass_backtrace.first[:line]
         raise e
       end
+
+      private
+
+      def importer_class
+        SassImporter
+      end
     end
 
     class ScssTemplate < SassTemplate