Add Support for Rejecting a PR

[x3dfxjunkie]

I have a use case where some workflows only apply if submitting code files (.py, .js, etc). There is an abbreviated workflow for documentation-only changes (.md, etc). Because of this, I can't "require" my workflows by name, just that any that report in aren't failing. There is also a bit of a race condition, it seems, where a PR can be merged when workflow jobs are running, but haven't report pass/fail.

With this in mind, I'd love to see it support rejecting PRs. My proposed workflow with that:

1. Start workflow
2. Reject PR
3. Tests
4. Coverage
5. Build
6. ...
7. Approve PR