Add support for differential field specifications to be indexed in search

Summary: ...and do so for a few fields -- summary, test plan, and revert plan.

Test Plan: added NATASHA and BULLWINKLE to summary and test plan of existing diff. Diff showed up in search!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T654

Differential Revision: https://secure.phabricator.com/D3915