Flow: Add an option to require minimum coverage

[gajus]

Add an option (such as --minimum-coverage <number %>) to flow check that would require code coverage to be equal or greater to the set threshold. If threshold is not set, exit with exit code 1.

This would be useful for precommit and CI integration.

[mroch]

I've thought about having flow check and flow status report coverage. I think that'd be more flexible, and your commit hook or CI integration could error on its own.

[MoOx]

https://www.npmjs.com/package/flow-coverage-report can help with that, it offers a threshold option!