## ButtonGroup
使多个按钮平成一行

### 基本使用
``` html
    <a-button-group>
        <a-button type="primary">primary</a-button>
        <a-button type="white">white</a-button>
        <a-button type="info">info</a-button>
    </a-button-group>
    
    <!-- column 让按钮垂直排列 -->
    <a-button-group class="gutter-top" column>
        <a-button type="white">white</a-button>
        <a-button type="white">white</a-button>
        <a-button type="white">white</a-button>
    </a-button-group>
```