Makes Gollum adhere to formatter limitations set in config.rb.

Gollum::Markup.formats now adheres to formatter limitations from config.rb. Fixes issue https://github.com/gollum/gollum/issues/771.
![gollum_issue-771](https://f.cloud.github.com/assets/571173/1935479/32d013b8-7efa-11e3-9068-43dfa04b4ec4.png)