Add namespace option

Citing https://github.com/symfony/symfony/pull/8639#issuecomment-22385631 and the discussion there. Sorry for being a bit late.

On that note, it appears the MongoDBBundle already has this option available: https://github.com/doctrine/DoctrineMongoDBBundle/blob/master/DependencyInjection/Configuration.php#L151 (Correct me if I am wrong)

This makes the bundle able to use a custom namespace for each cache driver.