Add option for additional classes on nav element

This includes an option now to _add_ classes to the containing `nav` element. We could make the option to replace the `scroll-nav` class altogether, but that would really mess up the BEM convention of the class structure of the whole element. Adding classes keeps the convention intact and the element differentiated from other items on the page while allowing inclusivity for styling or whatever.

This is a place to start though. The test I created seems pointless, but I wasn't sure how to do unit testing on an option the dev could set when calling the function.