The router API should allow its backend to be configurable

[dead-claudia]

It should be possible to configure how URLs are routed, so people aren't having to fork it for use cases like this. For comparison, it'd give us the ability to provide a hook like React Router's history attribute, without much work on our part.

(It's come up on Gitter more than once. Marking for v2 because it's a pretty trivial change on our part.)

Here's what I propose:

• Instead of accepting a mock $window directly for the core router, accept a backend, and default to one that wraps $window. Same goes for the API frontend.
• For the backend, require the following methods and use them in lieu of the given blocks:
  • backend.normalize(fragment) - Get and normalize the specified fragment.
  • backend.setPath(prefix, path, state, title, replace) - Set the path given certain options
  • backend.initialState() - Get the initial state to potentially assign.
  • backend.registerResolver(resolver) - Register the router's resolveRoutes function.

Basically, I don't want to require every user that needs to override the underlying routing mechanism to redo the entire router, but instead just require them to implement a few methods.

[dead-claudia]

If I find time, I'll implement this. It's pretty straightforward.