Align aggregate api with mongo docs without breaking

[miccarr]

Changes the db.collection.aggregate() api to accept both the custom aggregate( step1, step2, …) and the official mongo aggregate( pipeline, options ) notations.

The aggregate() should accept the pipeline as a string, allowing to pass options object. Does the same as #41 , but keeping compatibility with current notation.

Mongo docs for reference: https://docs.mongodb.org/manual/reference/method/db.collection.aggregate/

[jordymeow]

This merge would be so welcomed. Is actually promised-mongo abandoned?