Issue #13 -  Get parts of content independently

New feature added as @bradvogel suggested in #13:

```javascript
var extractor = require('unfluff');
var everything = extractor(html);
var justTitle = extractor.title(html);
```
The cleaner isn't idempotent. I actually changes the document's DOM and could continue to change it each time. 

So calling it multiple times will not work. That means something like this will break wildly and unpredictably because it re-cleans and re-finds the top node each time:

```javascript
var doc = extractor.from(html);
var text = doc.text();
var videos = doc.videos();
```

It's important that the cleaner and top node search is only done once for a document.  It's something that should be a dependency of calculating the text/videos but not something that's done multiple times. So this will require some smarter logic.