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);
```
I acutally had a think about this and had different approaches. 
**First** is to use a straightforward approach and cache created doc in in something like 
```coffeescript
extractor.title = (html) ->
  title(extractor.getCachedDoc(html) || cheerio.load(html))
```
The **second** is to slghtly change the API you are requesting. See usage:

var extractor = require('unfluff');
var everything = extractor(html);
//but
var nothingIsExtractedYet = new extractor(html);
console.log(nothingIsExtractedYet.title());
//or
var nothingIsExtractedYet = extractor.from(html);
console.log(nothingIsExtractedYet.title());

I prefer **second** approach. What do you think?