allow rendering ActiveField without surrounding tag

Example use case:
In tabular input I want to put two fields in one `td` for example quantity and a dropdown for quantity unit:

```php
<tr>
    <?php $options = array(
        'tag' => 'td',
        'template' => "{input}\n{error}",
    ); ?>
    <?php echo $form->field($item, "[$i]posNr", $options)->textInput(); ?>
    <td>
        <?php echo $form->field($item, "[$i]quantity", array(
            'tag' => false,
            'template' => "{input}\n{error}",
        ))->textInput();
        echo $form->field($item, "[$i]quantityUnit", array(
            'tag' => false,
            'template' => "{input}\n{error}",
        ))->dropDownList($item->getQuantitiyUnits()); ?>
    </td>
    <?php echo $form->field($item, "[$i]description", $options)->textarea(); ?>
</tr>