Gollum::Markup.formats now adheres to formatter limitations from config.rb (with tests). Fixes issue https://github.com/gollum/gollum/issues/771.
Merge pull request #56 from repotag/limit_formatters

Makes Gollum adhere to formatter limitations set in config.rb. Resolves issue https://github.com/gollum/gollum/issues/771 .