allow deep merging

If two bundles want to enable one listener each by prepending extension
configuration, let them.
@see
https://github.com/stof/StofDoctrineExtensionsBundle/commit/7705f653eb1140dc86c9942b381bda30126e21fe#commitcomment-3944254
Merge pull request #211 from greg0ire/deep_merging

allow deep merging