Added PlainText filter to render .txt files in <pre></pre> block without additional markup (with tests). Resolves https://github.com/gollum/gollum/issues/775 .
Merge pull request #55 from repotag/plain_text_formatter

Added PlainText filter to render .txt files in <pre></pre> block without additional markup (issue https://github.com/gollum/gollum/issues/775).