colspan option

add colspan option for asymmetrical forms

for example:
```php
echo Form::widget([
    'model' => $model,
    'form' => $form,
    'columns' => 3,
    'attributes' => [
        'name' => ['type'=>Form::INPUT_TEXT, 'columnOptions'=>['colspan' => 2]],
        'age' => ['type'=>Form::INPUT_TEXT],
    ]
]);
```