Add option to clear out Launch URL after requesting it

After launching the app via a custom url scheme, that URL is persisted on the `window` object until the app is closed. This causes it to attempt whatever action is supplied in the URL every time it's reopened and a `resume` listener is set. As you can imagine, I'd only like my app to execute whatever action is supplied once, and then forget about it. 

I provided this fix as an option to the `getLaunchURL()` action as to not add breaking changes to anyone's existing app.

Let me know if you have any questions, thanks!