Added params to user defined settings fields

It allows to create own function which will display all custom fields based on params from additional array, i.e.
array(
    'name' => 'Expiry Date,
    'id' => 'exp_date',
    'type' => 'user_defined',
    'display' => 'getUserDefinedFields',
    'params' => array('date', 'exp_date', 'rrrr-mm-dd') //
);

function getUserDefinedFields($option, $id, $placeholder){
    switch ($option){
        case 'date' : 
            return '<input type="text" id=".$id.'" name="'.id.'" class="date-pick" value="" placeholder="'.$placeholder.'" />';
        }
}