Implement aggregation endpoint

[EugeneButusov]

For flexibility, I'd like to add aggregation endpoint. It's safe, because no writing operations happen, and would be useful for client-side: create groupBy operations, some additional flexible stuff provided by mongodb.

Some questions must be resolved:

• data-source dependency: aggregate is not db-agnostic purely. Need to decide how to name it.
• how to implement that: action or additional param, like a filter.
• exact methods to handle aggregation process in controller.