Make "arc patch" read `authorName` and `authorEmail`

Summary: If provided, have `arc patch` use `authorName` / `authorEmail`. This simplifies handling and makes patches more portable between version control systems (previously, information was generated in the diff's VCS, regardless of which VCS it was being applied to).

Test Plan: Created a diff with author `derp <derp@derp.com>`, ran `arc patch --diff x`, got a local commit with that author.

Reviewers: btrahan, edward, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4827