Initial :lipstick: for comments with markdown support

Card: http://catchup.io/boards/3/cards/78

@as-cii would you mind? :wink: 
Maybe we could even use namespaces here: `Redcarpet::Render::HTMLComment` or `Markdown::Render::HTMLComment`, but we should change how rails loads files and I am not in the mood to do this right now :stuck_out_tongue_winking_eye: