on(name, fn) {
    if (['start', 'stop', 'update', 'complete', 'pause', 'resume'].indexOf(name) > -1) {
      this[`_on${name.charAt(0).toUpperCase() + name.slice(1)}`] = fn;
    }
    return this;
  }
Method to add callbacks on the fly.
@param {string} name callback name
@param {Function} fn callback function
@returns {TweenExtra}