[ticket/12841] Allow extensions to position new config vars

https://tracker.phpbb.com/browse/PHPBB3-12841
This would be called by an extension using a command like:
```php
$display_vars['vars'] = insert_config_array($display_vars['vars'], $custom_config_vars, array('after' => 'config_name'));
```
That would place their new $custom_config_vars array after core's existing "config_name" ACP config. They could also use 'before' if needed too.

PHPBB3-12841