字符串最大长度验证

•  {title}占位符，对应columns里面title

 "title": [    
      {
        "type": "maxlength",
        "required": false,
        "isAdd": true,
        "isEdit": true,
        "max": 500,
        "message": "【{title}】值最大长度为500。"
      }
    ],