Add parse_icons function for prepend and append input group

Add the parse_icons function for prepend and append on input group.

Example :
```php 
->add('birthDate', 'text', array(
    'attr' => array(
        'class' => 'datepicker',
        'input_group' => array(
            'prepend' => '.icon-calendar'
        )
    )
))
```