Properly handle replies to emails

[eranrund]

Currently, when an email sent to fire@ is also CCd and someone reply-all to it, fire-bot will create a new issue instead of replying to the original one. It would be nice to make it reply.

[Josyann]

Hi! I would like to suggest an enhancement of this feature : comments on the github issue are mailed to the original mail. It would be awesome to have the full follow-up by mail :)