实现购物车的加减

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  <ul>
    <li>市场参考价格:<span>￥2000.00</span></li>
    <li>礼品编号：<span>SH001</span></li>
    <li>兑换时间：<span>无期限</span></li>
    <li>剩余数量：<span>998</span></li>
    <li>每个会员最多兑换：<span>无限制</span></li>
    <li>兑换数量：
        <div id="add-plus">
          <input type="button" class="addplus" value="-" />
          <strong>0</strong>
          <input type="button" class="addplus" value="+" />
        </div>
    </li>
  </ul>