code: function getHtmlFromPureMarkdown(srcStr) {
  	var html = md.render(srcStr);
  	var isFirst = true;
  	var newstr = html.replace(/<h[123](.*?)>/igm, function() {
  		var fullTag = arguments[0]  //'<h2 class="align-center" data-background-color="blue">'
  		var attr =  arguments[1] // class="align-center" data-background-color="blue"
  		var tag = fullTag.match(/<(h[123])/)[1]   // h2
  		var str = '<section data-tag="' + tag + '" ' + attr + '>' + fullTag;
  
  		if (isFirst) {
  			isFirst = false;
  			return str;
  		}
  
  		return '</section>'+str;
  	})
  	newstr += '</section>';
  	var $ = cheerio.load(newstr);
  	$('section[data-tag=h3]').each(function() {
  		$('<section data-tag="down"></section>').insertBefore($(this).prev('section[data-tag=h2]'))
  	})
  
  	$('section[data-tag=down]').each(function() {
  		$(this).next('section[data-tag=h2]').appendTo($(this));
  		$(this).nextUntil('section[data-tag=h2], section[data-tag=down],section[data-tag=h2]').appendTo($(this));
  	})
  
  	return $.html();
  }
docs: 把纯净版的 markdown 字符串转换成 slide html