Strip replies and signatures from emails

Most emails are cleaned up nicely by `griddler`, but the occasional reply text slips through when someone replies from a strange email client (or speaks another language).

There are also some folks who have fancy signatures and `griddler` makes no attempt to clean those out.

This PR adds GitHub's email parsing secret sause, [`email_reply_parser`](https://github.com/github/email_reply_parser), and scrubs all email bodies one last time before creating an entry.
:bulb: Sharing this knowledge here: https://github.com/thoughtbot/griddler/pull/176