allow data attribute access by Liquid on Convertible items

lib/jekyll/convertible.rb

@@ -170,5 +170,14 @@ module Jekyll
         f.write(self.output)
       end
     end
+
+    # Accessor for data properties by Liquid.
+    #
+    # property - The String name of the property to retrieve.
+    #
+    # Returns the String value or nil if the property isn't included.
+    def [](property)
+      data[property]
+    end
   end
 end