Element：el-tree 添加自定义图标icon

<el-tree
  class="filter-tree"
  :data="data"
  :props="defaultProps"
  :highlight-current="true"
  node-key="value"
  :default-expanded-keys="[2]"
  :current-node-key="currentId"
  @node-click="nodeClick"
  ref="tree">
  <span class="custom-tree-node" slot-scope="{node}">
    <span>
      {{ node.label }}<i :class="node.icon"></i>
    </span>              
  </span>
</el-tree>