Log Message:
youtube-dl: updated to 2018.03.20

youtube-dl 2018.03.20:

Core
[extractor/common] Improve thumbnail extraction for HTML5 entries
Generalize XML manifest processing code and improve XSPF parsing
[extractor/common] Add _download_xml_handle
[extractor/common] Add support for relative URIs in _parse_xspf

Extractors
[7plus] Extract series metadata
[9now] Bypass geo restriction
[cbs] Skip unavailable assets
[canalc2] Add support for HTML5 videos
[ceskatelevize] Add support for iframe embeds
[prosiebensat1] Add support for galileo.tv
[generic] Add support for xfileshare embeds
[bilibili] Switch to v2 playurl API
[bilibili] Fix and improve extraction
[heise] Improve extraction
[instagram] Fix user videos extraction