Allow access to element's attributes in exclusiveFilter callback

Sorry for the dodgy pull request before - got it right this time!

Essentially the idea here is to allow the definition of an exclusive filter like this:

```javascript
exclusiveFilter: function(frame) {
    return frame.tag === 'img' && !(frame.attribs.src || '').trim();
}
```

which would filter images with no `src`.

Note that I've used `_.cloneDeep` to prevent alteration of the `frame` from inside the callback, or calling its method(s).