Generates output and saves to given file

        :param filename File name
        :type str or unicode

def save(self, filename=None):
        """ Generates output and saves to given file

        :param filename File name
        :type str or unicode
        """
        if filename is None:
            raise IOError('Converter#save: Undefined filename')
        cnt = self.output()
        with (open(filename, 'wb+')) as f:
            f.write(cnt.encode('utf-8'))