Correct all issues mentioned in the review.
    
    According to the review, there were several issues that bothered
    reviewer. This commit fixes all the issues and makes the configuration
    file to be used as planned.
    
    Fix config file reading
    
    This PR fixes the configuration file handling. Before, it required
    the --config-file CLI option to be read. Now, we read the default
    configuration file if it exists and if it is valid, then update
    option taken from the CLI.
    We also discard the --config-file option because we do not wish
    to allow extra configuration files.
    
    Update README with documentation.
    
    Change Help messages to show [yes|no].
    
    Improve error message on invalid options.
    
    Replace dashes with underscores in configuration file keys.
    
    Some users might want to use the CLI dashed variants in the
    configuration file which would not work for the program.
    This addition replaces the dashes with underscores, so now
    it is up to the user to use a dashed or underscored version of
    the configuration file key - wrap_width and wrap-width.
    
    Both will correctly work.
    
    Merges: https://pagure.io/fedora-easy-karma/pull-request/51