Feature: define ignored PostgreSQL fields by using `__ignore__` as source

[owahab]

One of the issues we came across was trying to create a SERIAL primary key in PostgreSQL that won't be copied and should not be inserted as NULL while copying from MongoDB. This hack allows this behavior using a magical string __ignore__.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: owahab
:x: rkhater
_{You have signed the CLA already but the status is still pending? Let us recheck it.}