vue-router导航守卫

https://router.vuejs.org/zh/guide/advanced/navigation-guards.html#%E5%85%A8%E5%B1%80%E5%89%8D%E7%BD%AE%E5%AE%88%E5%8D%AB

更新一下个人最新理解：https://www.cnblogs.com/twinkleG/p/15314498.html