Add ability to introduce additional PHP restrictions

Right now, the module will blindly overwrite whatever code is in the visibility box with the og_block_visibility_check code. It would be cool if you could also add your own restrictions to the PHP code in addition to what's done by the module, for example:

if ($_GET['q'] == 'admin') {
  return TRUE;
}
if (module_exist('og_block_visibility')) {
  return og_block_visibility_check('aggregator', 'feed-1');
}