7.x 2.x helper content render

Render a string

```php
$render = at_container('helper.content_render');
$render->render('Hello Drupal!');
```

Render content with function callback

```php
$render = at_container('helper.content_render');
$render->render(array(
  'function' => 't', 
  'arguments' => array('Hello Drupal!')));
```