Add a way to exit(1) if global coverage does not satisfies a certain %

[MoOx]

Jest have this feature http://facebook.github.io/jest/docs/configuration.html#coveragethreshold-object and it would be nice to have a similar CLI arg to fail on CI :)

[rpl]

its current behavior is to exit(2) when the total coverage is lower than the threshold (the default is 80% and it can be tweaked using --threshold command line option).

There is no command line option to disable this behavior, mostly because checking the total coverage is its main goal (besides "generating some nice reports to be able to more easily fix the flowtype coverage issues" :-)), and unfortunately there is no support for detecting an fail on a negative trend of the total coverage across two versions... yet (but I'd really like to add this feature asap).

I've added some integration tests to catch any future regression in this behavior (and I will expand the integration tests further in follow-ups PRs) in #60 (the current behavior is described by the commit jest snapshots: https://github.com/rpl/flow-coverage-report/pull/60/files#diff-ea5f4745f50d2aba384d45c0092557e2)

@MoOx let me know if an exit(2) on coverage failure is still an issue in your use case.

[benadamstyles]

unfortunately there is no support for detecting an fail on a negative trend of the total coverage across two versions... yet (but I'd really like to add this feature asap).

Do you mean something like https://github.com/Koleok/jest-coverage-ratchet? I use that and it's excellent, may provide some inspiration!