MongoDB 3.4+ supports a $replaceRoot stage while aggregating: https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/#pipe._S_replaceRoot

`lib/aggregation_cursor.js` seems to define prototype / class methods for method chaining instead of using an array/object-based raw pipeline. `.replaceRoot(

)` isn't available there yet and I'd like to add it to make our code (which uses chaining elsewhere) consistent.

I would be happy to submit a pull request for this, the only thing I was unsure about was how to limit the command to servers of version 3.4+ only, and whether this is necessary. Can someone advise me on this?