Bless Gollum with the power of GFM

With these changes, the Markdown rendered in Gollum wikis will have the
same quality and safety as the MD we render everywhere else @ GitHub.

This commit also changes the old `markup_class` accessor to
`markup_classes`, allowing users to specify a custom Markup class for
each markup language.

lib/gollum/page.rb

@@ -168,7 +168,7 @@ module Gollum
     #
     # Returns the String data.
     def formatted_data(&block)
-      @blob && @wiki.markup_class.new(self).render(historical?, &block)
+      @blob && @wiki.markup_classes[format].new(self).render(historical?, &block)
     end
 
     # Public: The format of the page.