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

This PR adds a PlainText filter in order to render .txt files in <pre></pre> block without additional markup (issue https://github.com/gollum/gollum/issues/775). Two things to note:
1) This PR changes the default handling of .txt files in Gollum. They will now be treated as pages instead of files.
2) The PlainText filter is applied in the filter chain after the meta-data filter. Code blocks, tags and other markup will not be parsed if the page is a .txt file. Please review before merging.