Integrate MIMA

Ok, this is kinda of a terrible one.  It's a first attempt at integrating MIMA's `previous-artifact` setting with sbt-release.  On release, first `mima-report-binary-issues` is run; if that fails it fails the process.  Then, after the release version has been assigned but before it's committed, the new release version is also used to set `previous-version` for future use.  Finally, in ALL projects where `previous-version` is not `None`, the `mima-previous-artifact` setting is created.

I hope this works.  There's a non-zero chance it won't, for multi-project builds.