feat[Menu]: menu icon support el-icon(#3048)

src/router/index.js

@@ -25,7 +25,7 @@ import nestedRouter from './modules/nested'
  * meta : {
     roles: ['admin','editor']    control the page roles (you can set multiple roles)
     title: 'title'               the name show in sidebar and breadcrumb (recommend set)
-    icon: 'svg-name'             the icon show in the sidebar
+    icon: 'svg-name'/'el-icon-x' the icon show in the sidebar
     noCache: true                if set true, the page will no be cached(default is false)
     affix: true                  if set true, the tag will affix in the tags-view
     breadcrumb: false            if set false, the item will hidden in breadcrumb(default is true)
@@ -197,14 +197,14 @@ export const asyncRoutes = [
     name: 'Example',
     meta: {
       title: 'Example',
-      icon: 'example'
+      icon: 'el-icon-s-help'
     },
     children: [
       {
         path: 'create',
         component: () => import('@/views/example/create'),
         name: 'CreateArticle',
-        meta: { title: 'Create Article', icon: 'edit' }
+        meta: { title: 'Create Article', icon: 'el-icon-s-help' }
       },
       {
         path: 'edit/:id(\\d+)',