Ability to inject query builder into provider

Hi, 

By adding this it would be possible to update the index by a complex query.

```php
$this->get("fos_elastica.provider_registry")
    ->getProvider('my_index', 'my_type')
    ->populate(
        null, 
        array(
            'query_builder' => $this->getDoctrine()
                ->getRepository('myModel')
                ->createQueryBuilder()
                ->field('fancy')->equals('yes')
        )
    );
```