Rethink the theme package system

[jacobobryant]

After #56 (and probably also #21) are complete, we should think about what the best way to publish/share/install themes would be. Right now the :site/theme field is a string which refers to a directory under themes/, e.g. default -> themes/default/. It would probably be better if it was a URL to a git repository. Maybe include a hash too, or just have platypub check for new versions periodically. Whenever you preview or deploy a site, Platypub would check if the repo has been downloaded already and clone it if not.

Maybe we could even have a directory + search for themes within platypub? e.g. if you tag your github repository with platypub, then it'll show up in the directory. and there's nice UI for installing/selecting themes so it'll feel natural for non-technical people.