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!')
));
```

Render content with a Twig template

```php
$render = at_container('helper.content_render');
$render->render(array(
  'template' => '@my_module/templates/hello.html.twig', 
  'variables' => array('name' => 'Drupal')
));
```

Render content with class

```php
$render = at_container('helper.content_render');
$render->render(array(
  'controller' => ['Foo', 'bar'],
  'arguments' => array('name' => 'Drupal')
));
```

Render content with Twig string template

```php
$render = at_container('helper.content_render');
$render->render(array(
  'template_string' => 'Hello {{ name|t }}', 
  'variables' => array('name' => 'Drupal')
));
```

Cache rendered-output

If the output is expensive, we can cache it:

```php
$render = at_container('helper.content_render');
$output = $render->render(array(

));
```