code: function modifyHTML(page, operations) {
      var html = page.getContent();
      var $ = cheerio.load(html);
  
      return Promise.forEach(operations, function(op) {
          return op($);
      })
      .then(function() {
          var resultHTML = $.html();
          return page.set('content', resultHTML);
      });
  }
docs: Apply a list of operations to a page and
  output the new page.
  
  @param {Page}
  @param {List|Array<Transformation>}
  @return {Promise<Page>}