Add an option to inline diffs up to a certain size in emails

Summary:
We already generate patches, but currently attach them. Allow them to be inlined instead (optionally, up to a certain size).

Also allow selection between unified and git patches.

Test Plan: Set these options in my local config, sent out a diff.

Reviewers: btrahan, Makinde

Reviewed By: Makinde

CC: aran, epriestley

Maniphest Tasks: T874

Differential Revision: https://secure.phabricator.com/D1759