Added Less and CSS support, modified tests to test included templates

I've added templates for both Less and CSS.  I also added the ability to simply specify the path as `'scss'`, `'less'`, or `'css'`, which allows the user to choose one of the built-in templates easily.

I also rewrote the tests to be more modular.  The tests now test the built-in templates, rather than a custom one, which was out-of-sync with the built-in template.

Finally, I documented the new functionality in the README.