Add configurable menus

Hi @JedWatson,

I needed configurable menus for my current keystone project and thought it might be useful to provide them out-of-the-box. It's pretty basic, it simply shifts the hard-coded navigation items to a model and serves them in the admin UI, obviously the views etc. still need to be created. The main advantage is that content admins are able to add links to dynamically created content.

It does make the most sense when you got some kind of "Page" model. If you're interested I can add a PR for that as well.