Add support for labels other than the WIP prefix

[mauroservienti]

Would be interesting to introduce the same behavior that the WIP prefix provides with labels as well, so that labelling a PR with a WIP label will have the same effect.

[alrz]

This would be a lot more preferable than "edit the title to enable merge" experience. though the label of the choice could vary and not hard coded -- do-not-merge, personal-review-only, work-in-progress, etc all may be used based on preference.

[dmetzler1988]

I also prefer a solution like this, on which we can manually input some signs for check (separated with a comma as example for multiple sign constellations).