I don't think the project is ready for an optional `afterEach` here. Ideally, it would look for the `afterWhatever` methods of a handful of major js testing frameworks and call any that exist. Right now, this only works for Mocha (and even that's a bit of a hack). If we make this optional and someone uses it where `afterEach` is undefined, then they could end up polluting global namespaced objects.

So, for now, I think I'd rather have it through an error than (possibly) pollute other tests.