added ActionColumn::$buttonOptions

allows for configuration of the default buttons like this:

```php
[
	'class' => ActionColumn::className(),
	'template' => '<div class="btn-group text-center">{view} {update} {delete}</div>',
	'buttonOptions' => [
		'class' => 'btn btn-default btn-xs',
	],
],
```

close #7633

CHANGELOG.md

@@ -5,6 +5,7 @@ Yii Framework 2 bootstrap extension Change Log
 -----------------------
 
 - Bug #5984: `yii\bootstrap\Activefield::checkbox()` caused browser to link label to the wrong input (cebe)
+- Enh #7633: Added `ActionColumn::$buttonOptions` for defining HTML options to be added to the default buttons (cebe)
 
 
 2.0.3 March 01, 2015