Add a basic Remarkup document rendering engine

Summary:
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.

Reviewers: avivey

Reviewed By: avivey

Subscribers: mydeveloperday, avivey

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19251