allow alteration of the url within modal_noderef_field_links()

I'd like to be able to use this module in conjunction with quicksketch's nodereference_url. In order to do that I need to be able to alter the url of the links that are being generated within modal_noderef_field_links(). Attached patch suggests adding an alter specifically for this purpose.

This would allow any other module to implement something like:

/**
 * Impementation of hook_modal_noderef_field_links_url_alter().
 */
function nodereference_url_modal_noderef_field_links_url_alter(&$url) {
  $node = menu_get_object();
  $url .= '/' . $node->nid;
}