Add support for virtual filesystems.

I am attempting to integrate physfs with this library, and it has some troubles with nested resources.
In other words, when I load a map with a tsx file included, because the library uses fopen instead of a custom handler, you can't use it.

Is there a way to integrate a virtual filesystem, like adding some callbacks like "tmx_load_file_fn" or "tmx_load_tileset_fn" ?