Add callbacks for filtering tests.

This is not at all the approach that I wanted to take for this feature, but it's the simple choice and it works. Also testing this in an automated way is slightly awkward so I omitted tests (it works though). I'll roll a solution where we can actually assert behavior similar to the JUnit 4 module later.

Closes #20.
Nice. Was refactoring from my ideal solution to this one. Good eye.