Make signature problems opt-in

[dwijnand]

Set ThisBuild / mimaReportSignatureProblems := true to opt-in.

TODO:

• Add/adapt the README docs
• Add/adapt a scripted test

[dwijnand]

I'm thinking of maybe cutting 0.6.5 sooner with the regression fix (and to test out the Maven Central switch), and make this (and perhaps some other interesting changes) the interesting part of 0.7.0.

[dwijnand]

Happy to tweak and even revert, if you have any late concerns, @raboof! Just let me know.

[SethTisue]

Curious what the reasoning was here. (I have the sinking feeling I knew once, but forgot.)

This came up over at typelevel/scalacheck#696

[dwijnand]

Sure: the reasoning is that signature problem identification isn't perfect, so my impression of the response that it had when it was on by default is "wtf, mima you're wrong again" and more blindly excluding (building on mima's already lack of package private understanding etc). So I made it a power user's option, so you opt-in to having to think about it and managing its shortcomings.