Add configuration for filter parameters

This pull is related to https://github.com/doctrine/mongodb-odm/issues/899
It allows set filter parameters in config file.
Beacause that names are used in (SimpleFilter.php)[https://github.com/bigfoot90/mongodb-odm/blob/filter-parameters-config/lib/Doctrine/ODM/MongoDB/Query/Filter/SimpleFilter.php].
But maybe, as you say, it is better not hardcoding names. I will update pull request.