无法自定义message？

不支持自定义message，请参考https://github.com/jzaefferer/jquery-validation ，很多时候messages节点内提示的文案内容是自定义的

``` js
const loginRule = {
 rules: {
 username: {
 type: 'email'
 },
 password: {
 required: true
 }
 },
 messages: {
 username: {
 required: "请输入用户名.",
 email: "请输入正确的邮箱格式"
 },
 password: {
 required: "请输入如密码."
 },
 }
};
```
