在使用elementui的时候，实现给input框前面或后面添加图标

 <el-form-item prop="username">
<el-input
     ref="username"
     prefix-icon="el-icon-user"
     v-model="loginForm.username"
     placeholder="账户"
     name="username"
     type="text"
     tabindex="1"
     autocomplete="off"
/>
</el-form-item>

<el-form-item prop="username">
<el-input
     ref="username"
     suffix-icon="el-icon-user"
     v-model="loginForm.username"
     placeholder="账户"
     name="username"
     type="text"
     tabindex="1"
     autocomplete="off"
/>
 </el-form-item>