Parse a tag command.

def _parse_tag(self, name):
        """Parse a tag command."""
        from_ = self._get_from(b'tag')
        tagger = self._get_user_info(b'tag', b'tagger',
                accept_just_who=True)
        message = self._get_data(b'tag', b'message')
        return commands.TagCommand(name, from_, tagger, message)