message: add demo: pullupload
diff: <li>
  <switch-option name="pull down refresh" :value="pullDownRefresh" @update:value="val => pullDownRefresh = val"></switch-option>
  </li>
  + <li>
  + <switch-option name="pull up load" :value="pullUpLoad" @update:value="val => pullUpLoad = val"></switch-option>
  + </li>
  </ul>
  </div>
  <div class="demo">
  <div class="title sub">Demo</div>
  <div class="scroll-list-wrap">
  - <scroll-list :scrollbar="scrollbar" :pullDownRefresh="pullDownRefresh"></scroll-list>
  + <scroll-list :scrollbar="scrollbar" :pullDownRefresh="pullDownRefresh" :pullUpLoad="pullUpLoad"></scroll-list>
  </div>
  </div>
  <div class="methods">
  data() {
  return {
  scrollbar: false,
  - pullDownRefresh: true
  + pullDownRefresh: true,
  + pullUpLoad: true
  }
  },
  components: {
  