[Feature Request] Attribute,'label' of <el-tab-pane/> does not support VNode

### Existing Component
是

### Component Name
Tabs

### Description
当我想将tabs的单个标签的标签名增加图标或者气泡时，发现的属性label仅支持string,不支持VNode，希望新版本可以增加这个支持，或者增加solt的支持或者和ant-react一样可以支持属性extr。（When I tried to add or to the tab-name of individual tabs in , I found that 's attribute 'label' only supports string and not VNode. I hope the new version can add this support, or solt support, or the extr property like ant-react can.）