Before render default hook

The slim.before.render hook allows for PHP variables to be created and changed at any point throughout the life cycle, and handed off to the view template with the latest possible values.  A very common use case would be:
```php
$app->hook('slim.before.render', function () use ($twig) {
  $twig->addGlobal('session',$_SESSION);
});
```
A session value can change at any given point in the life cycle, and the view template will have the latest possible value.