Allow adding CodeApprove metadata via git trailers

[rockwotj]

See: https://git-scm.com/docs/git-interpret-trailers

I've switched to a new stacked PR workflow using https://github.com/getcord/spr/

It's pretty great, and it allows you to set the reviewers of a pull request using git trailers like: Reviewer: <github login>. It'd be nice if when I did something like: CC: <github login> as a trailer that code approve would recognize this and add the appropriate CC to the review (only needed on the first commit in a PR).

[samatcodeapprove]

I like this idea! That's a good feature request.