Support multiple template directories

theme_dir is now a list which contains the user provided directory if given plus the packaged theme directory.

This is a proof of concept to see if the idea is sound - needs a bit work probably, tests at least.

Fixes #129
I really ought to start calling these things out, but my main concern with `mkdocs` at the moment isn't that the codebase is lacking, but that it's documentation isn't up to par. If we're making functional changes like this we ought to be tracking those in the documentation as a priority. I don't see the documentation for `theme_dir` being updated at the same time as making the functional change, so should I be concerned we're letting the docs drift? (Sorry I know I'm not in much of a position to speak wrt this right now, but still)