title: How can NightmareJS return full html source code?
body: <p>I use nightmare to crawl a web-page, In the window, I can find all items are loaded, but can't get the some item's html code printed.</p>
  
  <p>Any suggestion?</p>
  
  <p>My code.</p>
  
  <pre><code>var Nightmare = require('nightmare');
  
  var nightmare = Nightmare({
     show: true,
     webPreferences: {}})
  
  nightmare
   .goto('http://music.163.com/')
   .viewport(1600, 900)
   .wait(5000)
   .evaluate(function() {
       return document.documentElement.innerHTML;
   })
   .end()
   .then(function(res) {
     console.log('source code:', res); // not all the html code printed!
   });
  </code></pre>