title: [Page Object]Parameter for section in nightwatch.js
body: <p>I have a single-page-app and list of order section within, like this :</p>
  
  <pre><code>&lt;form&gt;
     &lt;form-order&gt; // section
       &lt;ElementA&gt;
       &lt;ElementB&gt;
       &lt;ElementC&gt;
     &lt;form-order&gt;
       &lt;ElementA&gt;
       &lt;ElementB&gt;
       &lt;ElementC&gt;
     &lt;form-order&gt;
       &lt;ElementA&gt;
       &lt;ElementB&gt;
       &lt;ElementC&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>and in nightwatch page-object :</p>
  
  <pre><code> form-order:{
    selector:'div[class=form-order]',
    elements:{
       ElementA:'#ElementA',
       ElementB:'#ElementB',
       ElementC:'#ElementC',
       }
    }
  </code></pre>
  
  <p>When i call this section, there will be 3 of them,same selector,so the first one will be used.The question is how can i set :nth-child() for this section and elements within,it should be like this:</p>
  
  <pre><code> form-order:{
    selector:"div[class=form-order]:nth-child("+n+")",
    elements:{
       ElementA:"#ElementA",
       ElementB:"#ElementB",
       ElementC:"#ElementC",
       }
    }
  </code></pre>
  
  <p>But i dont know how to do it in nightwach.js or is there any solution for this?
  Thank for reading.</p>