Set a plugin option in configuration file.

        Note: Use sig_option_changed to call it from widgets of the
              same or another plugin.

def set_option(self, option, value):
        """
        Set a plugin option in configuration file.

        Note: Use sig_option_changed to call it from widgets of the
              same or another plugin.
        """
        CONF.set(self.CONF_SECTION, str(option), value)