def append_help_messages
      option << "Default: #{help_message_for_default}" if default
      option << "Value if no argument supplied: #{value_if_empty}" if value_if_empty
      option << 'This option is mandatory' if required?
      option << "This option is mandatory unless #{required_unless.join(' or ')} is/are supplied" unless required_unless.empty?
    end
def append_help_messages
      option << "Default: #{help_message_for_default}" if default
      option << "Value if no argument supplied: #{value_if_empty}" if value_if_empty
      option << 'This option is mandatory' if required?
      option << "This option is mandatory unless #{required_unless.join(' or ')} is/are supplied" unless required_unless.empty?
    end
@param [ Array ] option See OptionParser#on
 @param [ Hash ] attrs
 @option attrs  [ Boolean ] :required
 @options attrs [ Array<Symbol>, Symbol ] :required_unless
 @option attrs  [ Mixed ] :default The default value to use if the option is not supplied
 @option attrs  [ Mixed ] :value_if_empty The value to use if no argument has been supplied
 @option attrs  [ Array<Symbol> ] :normalize See #normalize

 @note The :default and :normalize 'logics' are done in OptParseValidator::OptParser#add_option
 @return [ Void ]