element select 多选中增加全选

<template>
  <el-select v-model="value" placeholder="请选择" multiple>
　　 
    <el-option :key="all" label="All" value="all" :disabled="value.length > 0 && value.indexOf('all') < 0">el-option>
　　 
    <el-option :disabled="value == 'all'"
      v-for="item in cities"
      :key="item.value"
      :label="item.label"
      :value="item.value">
      <span style="float: left">{{ item.label }}span>
      <span style="float: right; color: #8492a6; font-size: 13px">{{ item.value }}span>
    el-option>
  el-select>
template>

测试地址
https://codepen.io/anon/pen/dBPqbX?&editable=true
将代码贴入html id为app的div中