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
The youtube test video failed with `UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 34: ordinal not in range(128)`, the problem was with the filenames being encoded.