[youtube] Download DASH manifest

If given, download and parse the DASH manifest file, in order to get ultra-HQ formats.
Fixes #2166
[xvideos] Fix thumbnail extraction

Signed-off-by: Philipp Hagemeister <phihag@phihag.de>
[xvideos] Simplify
Merge branch 'youtube-dash-manifest'

Conflicts:
	youtube_dl/extractor/youtube.py