Log Message:
youtube-dl: Update to 20181231

Changes:
version 2018.12.31
Extractors
+ [bbc] Add support for another embed pattern (#18643)
+ [npo:live] Add support for npostart.nl (#18644)
* [beeg] Fix extraction (#18610, #18626)
* [youtube] Unescape HTML for series (#18641)
+ [youtube] Extract more format metadata
* [youtube] Detect DRM protected videos (#1774)
* [youtube] Relax HTML5 player regular expressions (#18465, #18466)
* [youtube] Extend HTML5 player regular expression (#17516)
+ [liveleak] Add support for another embed type and restore original
  format extraction
+ [crackle] Extract ISM and HTTP formats
+ [twitter] Pass Referer with card request (#18579)
* [mediasite] Extend URL regular expression (#18558)
+ [lecturio] Add support for lecturio.de (#18562)
+ [discovery] Add support for Scripps Networks watch domains (#17947)