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);
```
PS: This is part of why I didn't offer a per-attribute API. The module is more of a pipeline then a set of independent operations that can be run at any time. So it will take some cleverness to make that work without making the code super complex.