Log Message:
youtube-dl: Update to 20181217

Changes:
version 2018.12.17

Extractors
* [ard:beta] Improve geo restricted videos extraction
* [ard:beta] Fix subtitles extraction
* [ard:beta] Improve extraction robustness
* [ard:beta] Relax URL regular expression (#18441)
* [acast] Add support for embed.acast.com and play.acast.com (#18483)
* [iprima] Relax URL regular expression (#18515, #18540)
* [vrv] Fix initial state extraction (#18553)
* [youtube] Fix mark watched (#18546)
+ [safari] Add support for learning.oreilly.com (#18510)
* [youtube] Fix multifeed extraction (#18531)
* [lecturio] Improve subtitles extraction (#18488)
* [uol] Fix format URL extraction (#18480)
+ [ard:mediathek] Add support for classic.ardmediathek.de (#18473)