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);
```
I like the first, though you probably want a private function that keeps a hash of parsed cheerio docs, e.g.:

```
extractor.title = function(html){
   return title(getDoc(html))
}

function getDoc(html){
   var docs = {};
   if (!docs[html]) docs[html] = cheerio.load(html);
   return docs[html];
}
   
```

The second is break API change. Calls to `extractor(html)` will automatically generate text (as it does today), and we're trying to avoid that.