Fix --config handling and allow --append-config
From my testing, passing --config ./file.toml wasn't being handled correctly. Debugging it, it looks like parse_preliminary_options in flake8 strips out these options, so by the time we get to it in parse_configuration_and_cli they are nowhere to be found in argv.
@orsinium this is good to be reviewed
I tested the --config option and it now seems to work with this patch.
Thank you for your contribution! Sorry for postponing the review of the PR. I'm still not sure what PR does, how it fixes the issue. Probably, I should try to debug it myself as well.
W/o this patch, the --config option (and probaply the --apend-config option as well) does not work since it's stripped away by flake8 before flakehell has a chance to see it.