Add flexbox button groups (.button-grouped.flex)

Flexbox button groups (`.vertical` layout takes precedence):

```HTML
<div class='button-grouped flex'>
  <a class='button'>A</a>
  <a class='button'>B</a>
  <a class='button'>C</a>
</div>
```

I also updated the Kitchen Sink `#uibuttons` example.