Support email integration

We would like to support both sending and receiving emails for code reviews.

My current thought is, in order to do this, we should add wrappers around [git-format-patch](https://git-scm.com/docs/git-format-patch) and [git-am](https://git-scm.com/docs/git-am).

More specifically, I'm thinking that we could add two new subcommands: 'git appraise format-patch' and 'git appraise am', which would call into the corresponding git commands.

For 'git appraise format-patch', we would want to additionally turn comments into mail messages that quote the patch they are commenting on.

Correspondingly, for 'git appraise am', we would want it to read replies that quote a patch and turn the non-quote lines into inline comments.