Turns TextureMappings into JsonMappings which abstracts this system out to allow other asset types to take advantage of the JSON mapping system. This also brings some nice changes which makes working with the mappings easier, notibly, the JSON mapping is "compiled" before hand, which means while the JSON can have objects, at the end, you just need to input a path:

{
    "block": {
        "test": "success"
    }
}

turns into

{
    "block/test": "block/success"
}

For textures, the sub directory changing is handled after the fact still (block -> blocks), so this has been handled.