Fix a problem with parsing quiet and verbose options specified in config files

I've found a problem where specifying the quiet option in config files does not work as documented at http://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-q. I want to disable the option by specifying quiet = 0 in a local config file and when I did that, I got no output from flake8 just like specifying quiet = 2 or -qq.

This merge request is for fixing the problem.

BTW, I thought this could also fix the same problem with the verbose option, but actually it couldn't. Even after fixing the parsing problem, specifying the verbose option in config files, such as verbose = 2, seems to have no effects and shows no verbose logs. I guess there are some other problems with the verbose option handling (it seems flake8.configure_logging is called only before parsing config files, etc.), but such problems should be addressed in a separate issue.