Add a basic Remarkup document rendering engine
Ref T13105. Although Markdown is trickier to deal with, we can handle Remarkup easily.
This may need some support for encoding options.
Test Plan: Viewed .remarkup files, got remarkup document presentation by default. Viewed other text files, got an option to render as remarkup.
Reviewed By: avivey
Subscribers: mydeveloperday, avivey
Maniphest Tasks: T13105
Differential Revision: https://secure.phabricator.com/D19251