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);
```
No need to blame it, I did Ruby a little, it's great. I just thought that `@field` notation is only used when using `class` in Coffeescript.