R1:b71a55900aca

Allow tweaking of Differential mail by using events

Summary: Allow tweaking Differential mail before sending.

Test Plan:
Wrote a listener renaming Differential attachments and it worked without
problems.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, mareksapota, davidreuss

Differential Revision: 1091