`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
For consistency, I'd also vote for returning `true` -- if developer opts out of this behavior, then `navigate()` truly does nothing with the URL, and would return `false`. Also, if the developer expects a route may fall through to `window.location` then s/he probably (maybe?) would not be expecting to do much with the return value, since the page context will be blown away.