[fw-general] $this->addElement Validators with optional params ?

[J DeBord]

How would I set the optional value for the validator Alnum to true using the
addElement syntax below?

$this->addElement('text', 'l_description', array(
            'decorators' => $this->_defaultDecorators,
            'label'        => 'Link Description',
            'required'    => false,
            'filters' => array('StringTrim'),
            'validators' => array('Alnum')
        ));

Thanks.