`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 went back and forth on this. Ended up with a shorter option name (`handleFallThrough`) since it's being sent to the `navigate()` method, but I'm open to either. Will let the YUI team make the call on this.