Consider shipping a ponyfill, or the ability to override the native version #63

Hi! I've been testing this polyfill across different browsers and screen readers. What I've found is that the polyfill works really well, but now that the latest Chrome and FireFox ship ariaNotify natively, the polyfill isn't always active. This only seems to cause an issue with JAWS, which just does not work with the native ariaNotify, but does work with the polyfill. For consistency across browsers, I'd like to be able to enable the polyfill even if ariaNotify is present. Or even better, expose a ponyfill as well so we can call that directly and not have to overwrite the native method.