[Bug Report] tabbar-item使用了icon-class 属性设置icon不生效

### VUX version
2.9.1

### OS/Browsers version
macOS / Google Chrome 66.0.3359.181 （64位）/Vue 2.9.3

### Vue version
2.5.17-beta.0

### Code

```html

import { Tabbar, TabbarItem, Group, Cell } from 'vux'

export default {
components: {
Tabbar,
TabbarItem,
Group,
Cell
}
}

```

### Steps to reproduce
1、项目statci下导入iconfont.css文件
2、index.html引用文件

### What is Expected?
正常显示icon

### What is actually happening?
没有显示icon