handle downloads of videos from youtube playlist

It looks like youtube playlists aren't handled at the moment. This should return an array or videos if playlist have been passed and lists of formats for all the videos in the playlist.

```js
var video = youtubedl('https://www.youtube.com/playlist?list=PLtl803dIwdQlNse-M9TQxWEguwTgEs3Gl', ['--max-quality=18']);
``` 

Did not have a time to implement [download cue](https://github.com/fent/node-youtube-dl/blob/master/lib/youtube-dl.js#L30-64) yet