add committer as supported field for herald rules

Summary: Allows building Herald rules against committer, similar to author. Useful for monitoring cherry-picked commits.

Test Plan: Applied patch, restarted php-fpm and phd daemons to ensure code changes took effect. Added a new herald rule to trigger audit when committer was me. Cherry-picked someone else's commit (author=them, committer=me) and pushed to origin. Audit was triggered.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6838