生成目录html
        :return:
        :rtype:

def _gen_toc(self, idx):
        """
        生成目录html
        :return:
        :rtype:
        """
        if not self._chapterization:
            return ''

        start, end = self._start_end_of_index(idx)
        chap_infos = '\n'.join(["            <li><a href=\"#ch{}\">{}</a></li>".format(
            chap._idx,
            chap._title) for chap in self._chapters[start:end]])
        toc = """
    <div id="toc">
        <h2>
            目录<br />
        </h2>
        <ul>
{}
        </ul>
    </div>
    <div class="pagebreak"></div>
        """ .format(chap_infos)
        return toc