Add title.

        :param title: title for the current document
        :type title: string

        :param subtitle: subtitle for the current document
        :type subtitle: string

        :param source: source for the given title
        :type source: string

def add_title(self, title, subtitle=None, source=None):
        """Add title.

        :param title: title for the current document
        :type title: string

        :param subtitle: subtitle for the current document
        :type subtitle: string

        :param source: source for the given title
        :type source: string
        """
        title_entry = self._sourced_dict(
            source,
            title=title,
        )
        if subtitle is not None:
            title_entry['subtitle'] = subtitle

        self._append_to('titles', title_entry)