Allow add collections at runtime

Example:
     
```php                                                                                                                 
<?php                                                                                                                  
public function onBootstrap(MvcEvent $e){                                                                              
    if ($e->getRequest() instanceof HttpRequest){                                                                      
        $sm = $e->getApplication()->getServiceManager();                                                               
        $sm->get('AsseticBundle\Service')->getEventManager()->attach('setupRenderer', array($this, 'configureAssets'));
    }                                                                                                                  
}                                                                                                                      
```                                                                                                                    
                                                                                                                       
```php                                                                                                                 
<?php                                                                                                                  
public function configureAssets(EventInterface $e){                                                                    
     $response = array();                                                                                              
     /** @var \AsseticBundle\Service $target */                                                                        
     $target = $e->getTarget();                                                                                        
     $config = $target->getServiceLocator()->get('config');                                                            
     $config = $config['collections'];                                                                                 
     foreach($config as $assetName){                                                                                   
         if ($target->getAssetManager()->has($assetName)){                                                             
             $response[] = $assetName;                                                                                 
         }                                                                                                             
     }                                                                                                                 
     return $response;                                                                                                 
}                                                                                                                      
```