Added a method on FieldSpec to get all diff properties

Summary: We need to be able to query for all properties matching a given pattern, as mentioned in D3676

Test Plan: Loaded Phabricator, ensured diff loaded, ensured field using all properties was able to enumerate them.

Reviewers: vrana, epriestley

Reviewed By: vrana

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3679