Allow App::down() callback to be configurable based on app requirement. Fixes #1701.

With this changes, developer can actually add a whitelist structure:

```php

App::down(function  ()
{
    if (in_array($_SERVER['REMOTE_ADDR'], ['51.24.49.100', '34.245.99.39'])) return false;

    return Response::make("Be right back!", 503);
});
```

Signed-off-by: crynobone <crynobone@gmail.com>