allow custom template/static folders

This might not fit with the KISS idea of MarkWiki but I wanted to be able to specify custom template and static dirs to override the default theme. Templates are easy since you can just modify the jinja2 loader but static_folder requires either subclassing Flask or loading the user config early and passing a static_folder value to the Flask object initialization.

I haven't really looked into your frozen-flask related code yet so this commit might break that in new and interesting ways.

There's also a snippet offering an alternative method of implementing this but making it work is beyond my meagre OOP skills.

http://flask.pocoo.org/snippets/102/