Support for PR assignee

[davegaeddert]

The basic situation I see here is that people use the PR "assignee" to assign someone as a designated reviewer. There's a couple different ways this could work but I think the simplest is to use it like the author_approval setting -- the existing reviewer rules still apply, and then additional approval from the author (or assignee) could be required.

assignee_approval: required

[davegaeddert]

PullApprove v3 is now based off of the built-in GitHub reviewers, so I'm not sure assignees are relevant anymore. More info here: https://www.pullapprove.com