Allow users to add flags via Herald rules

Summary: Add "Mark with flag" rules to Herald.

Test Plan: Created / edited a "Mark with flag" rule. Parsed revisions / commits, got flags added.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley, vrana

Maniphest Tasks: T1041

Differential Revision: https://secure.phabricator.com/D2060