Add plugin system

[gkjohnson]

Related to #525, #490

class FadePlugin() {

  attach( tiles ) {

    // register events, etc

  }

  dispose() {

    // unregister events

  }

  async beginDataParse() {}
  async completeDataParse() {}

}

tiles = new TilesRenderer();

tiles.registerPlugin( new FadePlugin() );
tiles.registerPlugin( new IonPlugin() ); // GoogleAPIPlugin()
tiles.registerPlugin( new CustomMaterialPlugin() );
tiles.registerPlugin( new TextureOverlayPlugin() );
tiles.registerPlugin( new GLTFLoaderPlugin( { dracoLoader, meshOptDecoder } ) );