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);
```
It's a little more idiomatic in coffeescript to do `@doc_` instead of `this.doc_` (Using `@` instead of `this.`)

Blame Ruby :)