Add an abstract "Text" document engine and a "Source" document engine

Summary: Ref T13105. Allow normal text files to be rendered as documents, and add a "source code" rendering engine.

Test Plan: Viewed some source code.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19254