This commit allows to download pages in playlists as needed instead of all at once.
Before this commit,
youtube-dl http://www.youtube.com/user/ANNnewsCH/videos --playlist-end 2 --skip-download
took quite some time - now it's almost instantaneous.
As an example, the youtube:user extractor has been converted.
Fixes#2175
It needs to know the channel and the title of the list, because the ids change every time you browse the channels and are attached to a 'VISITOR_INFO1_LIVE' cookie.