[Feature Request] Ability to over-ride num-cores with argument

I'd like to be able to set the num-cores in my config file for builds but then over-ride it via an argument e.g. ./kdesrc-build --num-cores 4 neochat.

The case for this is when I'm playing games but I'd like to be able to multitask and do a build at the same time but limit the cores to x so it doesn't interfere with my game.