(main.js->template: '')替换 (index.html->div#app);
(index.html->) --> (components: { App })
( components: { App }) --> (import App from './App' -> src/App.vue)
(App.vue -> -> 路由组件) --> (main.js-> router) ========此项决定了页面展示那个组件内容 ========
({path: '/',name: 'HelloWorld', component: HelloWorld }) --> (import HelloWorld from '@/components/HelloWorld')
(src/components/HelloWorld.vue) -->
