register(callback) {
    if (this.pluginCallbacks.indexOf(callback) === -1) {
      this.pluginCallbacks.push(callback);
    }
    return this;
  }
Registers a plugin callback. This API is internally used to implement the various
glTF extensions but can also used by third-party code to add additional logic
to the loader.

@param {function(parser:GLTFParser)} callback - The callback function to register.
@return {GLTFLoader} A reference to this loader.