title: xml2js: can't set option in parseString() function
body: <pre><code>&lt;doc&gt;
    &lt;people&gt;
    &lt;name sex="male"&gt;Harry Potter&lt;/name&gt;
    &lt;age&gt;18&lt;/age&gt;
    &lt;/people&gt;
    &lt;people&gt;
    &lt;name sex="male"&gt;hermione granger&lt;/name&gt;
    &lt;age&gt;18&lt;/age&gt;
    &lt;/people&gt;
  &lt;/doc&gt;
  </code></pre>
  
  <p>From example xml I use xml2js to parse to object and I want to change the attribute name then I set the option in parseString() function but when I add any options in the function it doesn't work anymoe.</p>
  
  <pre><code>var xml2js = require('xml2js').Parser();
  
  xml2js.parseString(xml,{attrkey:'att'}, function(err, result){
      if(err) {throw err;}
      console.log(JSON.stringify(result));                        
  });
  </code></pre>
  
  <p>When I add option {attrkey:'att'} or others option to the function, it won't work. 
  Do I have something wrong ?</p>
  
  <p>Thanks for help !</p>