Restore ANSI highlighting of lint sections to lint output
Summary: Fixes T9846. This restores the last missing feature, ANSI highlighting of diff sections.
Added a mode so we can actually test this stuff, activated that mode, wrote unit tests.
Did a bunch of actual lint locally too and looked at it, all seemed sane.
Reviewed By: chad
Maniphest Tasks: T9846
Differential Revision: https://secure.phabricator.com/D18512