Other Auth plugin compatibility

[Gabo6480]

I'd like to have the ability to set up other auth plugins for user authentication. Maybe, instead of a straight AuthMe integration, an API could be implemented so we can add compatibility with SkinSystem to other plugins.

[Cyanoure]

+1
I would like to integrate it into my website with my own auth plugin without changing the SkinSystem's code.
For example it could have an option in the config file like "external_authentication" => true and only able to upload skin if the $_SESSION["username"] is not empty.

[Cyanoure]

Also, add the cache folders into the .gitignore file.