handle videos download from youtube playlist

If looks like youtube playlists aren't handled at the moment. This should return an array or videos if playlist have been passed. 

```
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
test update
download first if array of clips
typo
refactor
typo
Merge pull request #24 from przemyslawpluta/master

handle downloads of videos from youtube playlist