new variable 'management_custom_args'

First of all, thank you for this useful piece of code. We've been using and it has been amazing so far, but we needed something else and we would like to share with you.

This new variable allows you to have specific arguments when calling the management.main() method. This solves some problems when you have poor project structure, like the problem I had in the past days, and the only solution for that, at the moment, was forking and allow a custom option like this.

Obviously, this might not make sense to you guys, but it doesn't interfere with your builds anyway and it's only another option we will have.

Also, this is my first PR and I'm glad I can finally contribute with something to the open-source world.