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
Yup, good point. I had thought about this but then forgot. In terms of usage, it will only behave differently if you specify a ``theme_dir`` and set the ``theme`` to one built into MkDocs.

There are a few things I added in the release notes which need documented (i.e. Python 3, markdown extensions) - I'll do a sweep through those and update the docs.