Add describe.skip/only and it.skip/only

`skip` is only a wrapper around the already existing skip option, nothing much to say here.

`only` is more tricky, so feel free to review commits separately.
I've tried to be the least intrusive but I'm not sure you'll like me hooking onto the Lab object.
Anyway I went for a cleanup of the testing tree before running the tests because it feels like what we would want (as opposed to dozens of tests marked as skipped).

I hope that's more or less what you would have done.

Closes #110