Write default config file to disk.

        Backs up existing configuration file.

        Returns
        -------
        filename : string
            Path to config file.

def create_default_config(self):
        """Write default config file to disk.

        Backs up existing configuration file.

        Returns
        -------
        filename : string
            Path to config file.
        """
        filename = self.config_file.as_posix()
        logger.info("Creating default config file: %s", filename)
        config_dir = self.config_file.parent
        if not config_dir.exists():
            config_dir.mkdir(parents=True)
        if self.config_file.is_file():
            backup = filename + ".old"
            logger.info("Saving config file backup at: %s", backup)
            shutil.copy(filename, backup)
        with open(self.config_file, mode="w") as f:
            config_string = CONFIG_TEMPLATE.format(
                data_dir=self.default_config["data dir"],
                linewrap=self.default_config["linewrap"],
                colors=self.default_config["colors"],
                browser=self.default_config["web browser"],
            )
            f.write(config_string)
        return filename