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

src/docs/userguide/events.diviner

@@ -24,4 +24,11 @@ or alter the edit. Data available on this event:
     @{class:ManiphestTransaction}) being applied.
   - ##new## A boolean indicating if this task is being created.
   - ##mail## If this edit originates from email, the
-    @{class:PhabricatorMetaMTAReceivedMail} object.
+    @{class:PhabricatorMetaMTAReceivedMail} object.
+
+== PhabricatorEventType::TYPE_DIFFERENTIAL_WILLSENDMAIL ==
+
+This event is dispatche before Differential sends an email, and allows you to
+edit the message that will be sent.  Data available on this event:
+
+  - ##mail## The {@class:PhabricatorMetaMTAMail} being edited.