`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
> (Currently, the click handling code falls through to window.location if it doesn't route the click URL.

Sort of. It doesn't `preventDefault()` on the `click` event, which causes it to bubble up to the browser and is handled normally like any other anchor element. But I think it should be fine if instead it always went through `_navigate()`.