用keep-alive缓存路由组件

1、作用：让不展示的路由组件保持挂载，不被销毁

2、具体编码：

//下面的News和Message是组件名

//缓存一个用字符串
<keep-alive include='News'>
    <router-view></router-view>
</keep-alive>

//缓存多个用数组
<keep-alive :include=['News','Message']>
    <router-view></router-view>
</keep-alive>