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);
```
Yeah, that'd probably work. The only two requirements are that:

1. The existing API remains unbroken:
```
var extractor = require('unfluff');
var parts = extractor(html);
// parts.text
```

2. Calls to get the parts don't reparse the document:
```
var doc = extractor.from(html);
var title = doc.title();
var description = doc.description();
```

Then again, I defer to @ageitgey since I haven't contributed to this repo yet. I don't want to be a backseat driver :)