Disable warnings if --no-warnings is passed or they are disabled in config file

A colleague and me noticed that passing `--no-warnings` to linkchecker did not actually disable the warnings. I later also noticed that disabling them in the configuration file did not help either, the only way (apart from patching the code ;)) was to disable them in the configuration file *and* passing `--no-warnings`. It seems like this was broken since b38317d5 where optparse was replaced with argparse, and the change in this pull request seems to fix it.