Add a quoted body stripping pattern for "> On ... at ... wrote:"

Summary: See @scottmac's reply in T3982. It looks like his email client uses the standard quote string, but includes it in the quoted block.

Test Plan: Added a failing unit test, made it pass.

Reviewers: btrahan

Reviewed By: btrahan

CC: scottmac, aran

Differential Revision: https://secure.phabricator.com/D7440