[Feature Request] Support for automatic cache invalidation on http(s) images

There are many ways to check when to re-download a file. As far as I know, most servers now support the `If-Modified-Since` header and there is also `ETag` and `last-modified`. The library could do a HEAD request to the server and try to determine using any of these 3 methods if the file needs to be re-downloaded or not (as an option of course).

As far as I understand, in order to implement this myself, I have to either re-make the library or do some junky stuff (eg override some classes by copying code from here). Neither of these options seems good and I think this feature should be standard on these kinds of libraries as they're mostly used to fetch images from the web.

Thanks