Implement format selection in YoutubeDL

Now the IEs can set a formats field in the info_dict, with the formats ordered from worst to best quality. It's a list of dicts with the following fields:
* Mandatory: url and ext
* Optional: format and format_id

The format_id is used for choosing which formats have to be downloaded.

Now a video result is processed by the method process_video_result.

Implemented in TEDIE.
ArteTV: implement multiple formats