Provide an "algorithm" for stripping quoted text from email replies

Summary:
There's an undoubtedly-far-more-refined version of this in xmail if someone
wants to crib it for me. Otherwise we can anneal this as counterexamples arise.

This seems to be what mail.app and gmail do.

Test Plan:
Used mail receiver console to "send" some mail and verified it was correctly
truncated.

Reviewed By: jungejason
Reviewers: aran, tuomaspelkonen, jungejason
CC: aran, jungejason
Differential Revision: 290