Filter parameters config

This pull is related to https://github.com/doctrine/mongodb-odm/issues/899
I touched on this a bit in https://github.com/doctrine/DoctrineMongoDBBundle/pull/256, but I want to mimic ORM here. The primary issue in #899 was that we didn't have documentation for this at all, and you suggested allowing parameters (already part of the API) to be configurable in the bundle. That's all well and good -- we had existing functionality (parameters), but they weren't exposed in the configuration at all. Beyond that, though, I don't see value in having a SimpleFilter class for a few reasons:

 * It's less flexible than taking a single parameter (e.g. array, object) and merging that into the criteria; however, I'm not advocating that as anything I'd want to see in ODM.
 * In practice, I think it's better for users to create filters for specific classes or interfaces instead of some generic filter where everything is configurable. If we push all of the logic into the configuration, testing the filter class becomes rather pointless.
 * We already have a base class (instead of merely an interface) as a convenience for users to extend.

The latter two points are supported by ORM and its [documentation](http://doctrine-orm.readthedocs.org/en/latest/reference/filters.html#example-filter-class) (e.g. creating a filter for interfaces). Provided we have an example in the documentation, I don't think any of this is too complex for users to implement themselves.