adds subview support

```php
class Nested extends Mdogo_View
{
  function __construct($main = false, $sub = false)
  {
    parent::__construct($main);
    $this->sub = new Mdogo_View($sub);
  }
}

$view = new Nested('{{foo}} {{sub}}', '{{bar}}');
echo $view->render(new Mdogo_Model_Example(1));
```