Add `rake spec:units` and `rake spec:functionals`

Running the functional tests and unit tests together is kind of a pain since functionals usually take much longer (animations have to complete, etc).

These could be set on a project-by-project basis but I thought it'd be nice to have a sensible default. Took the nomenclature from Rails-land.

By default, these will run the tests in `"#{app.specs_dir}/unit"` and `"#{app.specs_dir}/functional"`, but those can be overridden by setting `app.unit_specs_dir` and `app.functional_specs_dir`. `rake spec` functions like normal.