Promise原理讲解 async+await应用（异步回调解决方案）

<! TOC "1.异步编程" "1.1.回调函数" "1.2 发布订阅" "1.3 观察者模式" "1.4 Promise" "2.promise用法与原理" "2.1 " "2.2 " "2.3 " "2.4 " "2.5 " "2.6 " "2.7 的一些扩展库" "2.8 应用 " "3.手