Allowing objects as filters

Hi,

Is there a particular reason why we can only add filter class names in the orm config and not objects?

For example when using the orm with symfony2 it would allow to use services as filters, currently I guess we have to manually get the filter from the em and set the needed dependencies.

If you're interested by this I can work on a PR in the orm to allow setting a filter with both an object or a class, and then retrieving or creating it in the FilterCollection enable method.

Bilal