http://gitunderground.i2p/archive/youtube-dl/src/commit/f28d7178e4afb58a94994796cd81c22183ca33c2/youtube_dl/extractor/common.py
The following fields are optional: alt_title: A secondary title of the video. display_id An alternative identifier for the video, not necessarily unique, but available before title. Typically, id is something like " 4234987 " , title " Dancing naked mole rats " , and display_id " dancing-naked-mole-rats " thumbnails: A list of dictionaries, with the following entries: * " id " (optional, string) - Thumbnail format ID * " url " * " preference " (optional, int)...