implement JetBrains IDE plugin

[schmonz]

The first implementation of Greencently (as a JUnit 5 extension) makes this tradeoff: in exchange for supporting only one test framework, it works no matter where or how you run the tests.

An IDE-plugin implementation could offer a different tradeoff: in exchange for running only within the IDE, it could work with any programming language and test framework the IDE supports.

For JUnit 5 users, there'd be no harm in using both. Either implementation could update the timestamp file and the pre-commit optimization would continue to apply.

This would open up Greencently to many more users. If they only ever run tests within the IDE, the pre-commit optimization will always apply.