Ability to provide a replacement `XHRLoader` for the loader plugin.

[Coderah]

Hi! I am using the Cache api (https://developer.mozilla.org/en-US/docs/Web/API/Cache) to have full control over caching and management of my 500mb+ game assets. However there is no feasible way (that I'm currently aware of or able to find) to have Phaser's normal asset system, which handles MultiAtlas and many other types quite well, be able to get files from something other than the internal XHRLoader. I was expecting File would reference the Phaser.Loader so I could at least override the xhr loader there and hack in my own approach.

Given that I was thinking it might be awesome to have the Loader plugin have a more generic approach allowing us to implement our own network (or otherwise) based loader handlers. I assume this might have implications on progress reporting and such. But most of it seems generic enough already, there's just no place to override the actual loader method.

If I can find the time I'll take a stab at a PR or at least API suggestion, but I'm not confident about my overall knowledge of the entire Loader system and would be afraid to do so without a lot of time to really grasp more.

[Coderah]

For reference, I just came up with an approach to add directly to the cache, and made a bunch of utils around that in my own code. but for things like MultiAtlas that meant re-implementing the logic it has to build out the correct data and images, which isn't terrible but still increases the work required for anyone to load assets outside of the standard XHR approach.