Set Default value for a given (section, option)
        -> called when a new (section, option) is set and no default exists

def set_default(self, section, option, default_value):
        """
        Set Default value for a given (section, option)
        -> called when a new (section, option) is set and no default exists
        """
        section = self._check_section_option(section, option)
        for sec, options in self.defaults:
            if sec == section:
                options[ option ] = default_value