实现 Promise finally

给 Promise 实现 finally 方法

废话不多说，比较简单，直接上代码吧

Promise.prototype.finally = function (callback) {
  let P = this.constructor;
  return this.then(
    value => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  );
};