Convert files to directory.
        :return: None
        :rtype: :py:obj:`None`

def _to_dir(self, file_generator):
        """Convert files to directory.
        :return: None
        :rtype: :py:obj:`None`
        """
        for f in file_generator:
            outpath = self._output_path(f.source, file_generator.to_format)

            if not os.path.exists(os.path.dirname(outpath)):
                os.makedirs(os.path.dirname(outpath))

            with open(outpath, mode="w") as outfile:
                f.write(outfile, file_generator.to_format)