Simplify router

I think that react-page is pretty hard to use right now. Specifically adding a client-side router that works well with the server-side one is the first challenge. This addresses it by removing virtually all of the routing logic and shifting the responsibility to the component author.

Basically, we always render `root.js` and pass an `initialPath` prop and the user can do what they want.

See https://github.com/petehunt/react-page/blob/new-rpm/src/root.js for what I mean.