Add pref to choose prefered markdown lib

lib/middleman/renderers/markdown.rb

@@ -1,12 +1,11 @@
-module Middleman
+module Middleman::Renderers::Markdown
-  module Renderers
-    module Markdown
   class << self
     def registered(app)
-          require "rdiscount"
+      app.set :markdown, ::Tilt::RDiscountTemplate
+      app.after_feature_init do
+        ::Tilt.prefer(app.settings.markdown)
+      end
     end
     alias :included :registered
   end
-    end
-  end
 end