Add effective_user as an option to the CLI config

I'm primarily using Snakebite for its CLI functionality and I think it's useful to be able to specify the `effective_user` in the `~/.snakebiterc`. This PR enables just that. I think it would also be nice to create a commandline option for that, but I didn't have time for that yet. If this PR is accepted, I'll see when I can add the commandline option as well :)