ytdl: When using DASH, print actual duration

prints the actual duration as reported by youtube-dl to the
terminal when available
ytdl: Try to handle multi-arc videos

this currently uses a sketchy but apparently working workaround,
which will be removed once the neccessary changes in youtube-dl
are implemented

Fixes #1277
ytdl: bump requrired youtube-dl version to 2014.11.26