Add useAriaLive hook

[dhovart]

I use react-aria-live in my current project, and it's working great so far!
But I find it tedious to have to use LiveMessenger and pass announcePolite and announceAssertive as props.
This hook makes it easier. If you're somewhere inside a LiveAnnouncer, you can simply do :

const { announcePolite, announceAssertive } = useAriaLive();

Note that this will only work with React 16.8+.