Ability to run a specific test suite and method when testing via the command line

I've been writing a lot of unit tests with Revel lately. One thing that was bothering me is when a specific test or suite was failing, I had to re-run the entire thing to see if the status changed. That not only took more time than I wanted, but often if a test was failing it would lock up the database and break the other tests anyway.

When I develop I wanted to continually re-run a specific test or test suite, as [Django allows you to do](https://docs.djangoproject.com/en/dev/topics/testing/overview/):
```b
# Run just one test case
$ ./manage.py test animals.tests.AnimalTestCase

# Run just one test method
$ ./manage.py test animals.tests.AnimalTestCase.test_animals_can_speak
```

With this pull request, you can. If you agree about this feature, I can modify gh-pages to reflect the new functionality.