Support core.test instead of midje

I use `midje-doc` in [replikativ](https://github.com/replikativ/replikativ) but would like to get rid of `midje` in favour of `core.test`. Could you add some examples of how to best do this and maybe also remove `(deftest test-name ... (testing "Description ...." ...))` and turn into some documentation markup instead of code? I guess the `(is (= ...))` forms aren't too bad then compared to the fact syntax. Does this make sense for you?