code: function tree (toc, tpl) {
    if ( tpl === void 0 ) tpl = '';
  
    if (!toc || !toc.length) { return '' }
  
    toc.forEach(function (node) {
      tpl += "<li><a class=\"section-link\" href=\"" + (node.slug) + "\">" + (node.title) + "</a></li>";
      if (node.children) {
        tpl += "<li><ul class=\"children\">" + (tree(node.children)) + "</li></ul>";
      }
    });
  
    return tpl
  }
docs: Render tree
  @param  {Array} tree
  @param  {String} tpl
  @return {String}