`navigate()` falls through to window.location on no matching route.

Per previous conversations with @ericf.
If this looks like an acceptable solution, I can add tests.

/cc @pdokas @jeremyruppel
I think we should look at all the places within the YUI App Framework where the return value from `navigate()` or `_navigate()` is used.

But I'm leaning towards removing the return value and this new option, and instead simply navigating to the URL. This will require some refactoring because we _still want_ the same-origin checks.