Add tlMimaPreviousVersions setting

[armanbilge]

We previously debated its usefulness in #46, but now I'm convinced.

1. mimaPreviousArtifacts is at the project level, but often you want to filter botched releases at the build-level. tlMimaPreviousVersions is build-level.
2. Some projects add extra versions, like cats includes the 1.x series for some modules.
3. Some projects are just weird, like scalacheck has this pvp version thing going on.

So yeah, I'm convinced :)

[danicheg]

Sorry for being late on this. Here are my 2 cents on the topic. Wouldn't that be confusing for users to deal with the settings behaving likewise the upstream settings? Say, the tlFoo setting where foo comes from an upstream library. An ability to configure things at the build/project level multiple ways increases entropy, I guess. Moreover, if new tl-* settings would slightly differ in behavior against original upstream settings, then it really can be hard to figure out things.

[armanbilge]

@danicheg thanks for your feedback, always much much appreciated!

I guess you are talking about the difference between mimaPreviousArtifacts and tlMimaPreviousVersions?

The problem is that mimaPreviousArtifacts is very difficult to configure correctly (remember that mouse had misconfigured this, so JS artifacts were being compared to JVM artifacts?). In fact many projects misconfigured this which is one of the reason I was motivated to work on sbt-typelevel.

I agree that it is confusing, and I would like to make this better though. WDYT?

[danicheg]

Nevermind. I'm just overthinking about corner cases, probably. sbt-typelevel can mitigate some difficulties of configuring plugins for their users. As you said, users might be wrong on it also without sbt-typelevel.

[armanbilge]

FWIW I expect this setting to be rarely used. Of all the Typelevel projects, really only Cats and Scalacheck seem to need it.

[armanbilge]

Darn, this PR introduced a regression.