Cancel pending image downloads when unmounted

When an image that is pending to be downloaded, is unmounted, the browser will still continue to download it. This problem is fixed in React-Image (https://github.com/mbrevda/react-image/pull/223).

As mentioned here: https://github.com/mbrevda/react-image

> On unmount React Image will abort any current image downloads potentially saving bandwidth and browser resources should the image no longer be desirable.

It would be interesting to also have this in React-Lazyload as it would otherwise harm performance.