Allow only and skip on `describe`.

[MicahZoltu]

As the test suite grows in test count, it can become time consuming to run all of the tests every time while you are iterating on a particular subset. It would be nice to be able to only run a specific describe set of tests at a time, or skip a particularly slow running set of tests without having to skip them all individually.

Also, if there are multiple should.only, only a single one (the last one) will actually run so even if you have parameterized tests you cannot constrain to a whole suite of them.