Add --score flag to set a minimal acceptance

[Felixoid]

Hello. Thank you for a great tool!

I like your library a lot and started to use it in my projects. Although, it's difficult to use it in CI without non-zero exit codes if the scores are non-acceptable.

Here's my attempt to implement the way to achieve both purposes at once.

[rjbordon]

oh this would be indeed helpful! any chance if / when this would be merged?

[Felixoid]

Awwwww, @zimmski, please :-j

[Felixoid]

I am eager to use go-mutesting in CI of https://github.com/Felixoid/braxpansion and https://github.com/Felixoid/coal-mine projects, but without the ability to exit with non-ok code it's useless, unfortunately =\