use ResizeObserver API when available

[lpillonel]

Any plan to use ResizeObserver when available ?
Currently supported by Chrome & Firefox.

[wnr]

That's a great idea. Would you like to create a PR?

[gilest]

Looks like an implementation is arriving in webkit too!

[wnr]

That's great! I've actually changed my mind. I think there should be a polyfill project, which could use this library as a fallback if the API does not exist natively. That way this library could get phased out in a smoother way.