feature[Mock]: mock-server support hot reload (#1850)

https://github.com/PanJiaChen/vue-element-admin/issues/1849

vue.config.js

@@ -41,22 +41,7 @@ module.exports = {
         }
       }
     },
-    after(app) {
-      require('@babel/register')
-      const bodyParser = require('body-parser')
-
-      // parse app.body
-      // http://expressjs.com/en/4x/api.html#req.body
-      app.use(bodyParser.json())
-      app.use(bodyParser.urlencoded({
-        extended: true
-      }))
-
-      const { default: mocks } = require('./mock')
-      for (const mock of mocks) {
-        app[mock.type](mock.url, mock.response)
-      }
-    }
+    after: require('./mock/mock-server.js')
   },
   configureWebpack: {
     // provide the app's title in webpack's name field, so that