Added encodeLabel param to \yii\grid\DataColumn

Use case:

```
    echo GridView::widget([
        ...
        'columns' => [
            ...
            [
                'attribute' => 'created_at',
                'label' => '<span data-toggle="tooltip" class="glyphicon glyphicon-time" title="'.Yii::t('app', 'Created At').'"></span>',
                'encodeLabel' => false,
                ...
            ],
```