Enable function formatResultCssClass.

Working on #1682 I noticed that a custom implementation of the function `formatResultCssClass` is never called as its behavior is always overwriten by:
 
```javascript
opts.formatResultCssClass = function(data) { return data.css; }; 
```

So I removed that line and made `return data.css` as the default behavior in the user options. This way it is possible to provide a custom implementation of `formatResultCssClass`.

Let me know if that works.