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, you're right about `new extractor` approach, I was little hasty.

What do you think about the alternative `extractor.from` thing? It should return an object with `title()`, `description()`, `text()` and other methods which would be called on demand and also keep cached version of cheerio doc in private field.

I'm just not pretty comfortable with adding a lot of methods to a function and keeping cache in a plain object.