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 

```
var doc = extractor.from(html);
```

approach seems like the best approach to me. It allows for future expansion and it doesn't require a messy api.

Overall, I want to keep this module incredibly dumb and simple to use.