Add ability to load spine from preloaded json and texture atlas

[f0nar]

I need ability to parse and construct spine from preloaded separate files. I don't use PIXI.Assets to load spine files so I need functionality to parse them by myself. In pixi-spine:6.x I was able to do it by exported SpineLoader. I think it would be great to do the same here.

I see that you have such point in examples but that is available in pixi-spine<=6.x https://github.com/pixijs/spine/blob/master/examples/preloaded_json.md

[f0nar]

@ivanpopelyshev @miltoncandelero sorry for ping but could you please check it? Really need it.