Add "Author" and "Committer" fields to Herald pre-commit content hooks

Summary: Ref T4195. Adds "Author" and "Committer" fields.

Test Plan:
Created a rule using these fields:

{F90897}

...then pushed git, mercurial and svn commits and verified the correct values populated in the transcript:

{F90898}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7802