Tab props accept vue component

Currently `tab` accepts only string. Since render function also accepts VueComponent, it would be great if `tab` also accepts component object. So I can avoid vuetify global component registration and import the vuetify component in `data` like this.
```vue
<script>
import { VList } from "vuetify/lib";
export default {
  data: () => ({ VList })
}
</script>
```