Add pref to choose prefered markdown lib

lib/middleman/renderers/markdown.rb

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