title: set the default value of TextField inside alertDialog
body: <p>i found the following sample code from Appcelerator's docs (<a href="http://docs.appcelerator.com/platform/latest/#!/api/Titanium.UI.AlertDialog-property-previewContext" rel="nofollow">http://docs.appcelerator.com/platform/latest/#!/api/Titanium.UI.AlertDialog-property-previewContext</a>):</p>
  
  <pre><code>    var dialog = Ti.UI.createAlertDialog({
      title: 'Enter text',
      style: Ti.UI.iOS.AlertDialogStyle.PLAIN_TEXT_INPUT,
      buttonNames: ['OK']
    });
    dialog.addEventListener('click', function(e){
      Ti.API.info('e.text: ' + e.text);
    });
    dialog.show();
  </code></pre>
  
  <p>The problem is that i need to set a default value for the input text, but the documentation only shows the possibility to set a placeholder, but not a default value for it.</p>