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);
```
Could you cache cheerio.load(html) so it's only called once for the common case of:

```
var extractor = require('unfluff');
var title = extractor.title(html);
var description = extractor.description(html); // Shouldn't reload the same document.
```