add support for Cloverage

[esuomi]

This is a continuation to #20 but with updated details:

• Cloverage is now capable of using external runners, see Allow external runners to be loaded from other dependencies. cloverage/cloverage#193
• as example, circleci.test has a simple extension ns for support itself: https://github.com/circleci/circleci.test/blob/master/src/circleci/test/cloverage.clj

[Deraen]

Another example here: https://github.com/metosin/bat-test/blob/master/src/metosin/bat_test/cloverage.clj

The difference between this and circleci.test is that,

With circleci.test one runs cloverage lein plugin which then calls into circleci.test to run the tests while Cloverage instrumentations are active. I'm not sure how well this approach works together with e.g. eftest report options. It could be hard to access plugin settings in the multimethod.

With bat-test the user runs bat-test like normal but with Cloverage option enabled, and in this bat-test takes care of enabling instrumentations and writing the output.

My approach requires duplicating some code from Cloverage, and it might make sense to add this API to Cloverage if another library wants to use this.