Very rough cut of prose diffs

Summary: Ref T3353. This is just sketching things out so I can wire them up. It probably doesn't do a particularly good job, but we can start refining it once it's hooked up the UI and we can catch problem cases.

Test Plan:
Ran prosediff.php, got reasonable looking results for a simple test case:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3353

Differential Revision: https://secure.phabricator.com/D16055