Add support for streaming/cursor

[EloB]

Thanks for an amazing library.

Is your feature request related to a problem? Please describe.
I'm planning to use this for big datasets and unsure how todo it. I know that pg has a sub package called pg-cursor that allows streaming data from postgres instead of having to read every row into memory.
https://node-postgres.com/apis/cursor

Describe the solution you'd like
If this already have supported would love to see some example of it else I did a PR that fixes #569.