对象形式的城市格式，转换成数组格式

    let city_list = City.city_list
    let province_list = City.province_list
    Object.keys(province_list).forEach((prop, index) => {
      let start = prop.substr(0, 2)
      this.cityArr[index] = {
        label: province_list[prop],
        id: prop
      }
      this.cityArr[index].children = []
      Object.keys(city_list).forEach((childrenProp, i) => {
        if (childrenProp.startsWith(start)) {
          let children = {
            label: city_list[childrenProp],
            id: childrenProp
          }
          this.cityArr[index].children.push(children)
        }
      })
    })