Make scooter work as a mixin library

Right now the only way to import styles from scooter is as classes. This means that the whole library will be imported if you import extend a class with one of scooters internal classes. If instead scooter started by providing placeholder classes `%classname` and then used those to create it's regular classes `.classname`, others could use scooter to extend their existing classes and not have to clutter up their markup with scooter specific class names