Adds looping audio.

Adds function necessary to do looping over audio files. Loops can happen one of
two ways:

* Primary nodes.
* Nodes by category.

To add a subnode to a category, simply add a "category" parameter with a number
for the category.

We'll still need to add interface buttons to play clips by category. We can
also, if desired, highlight the specific node/subnode being played.