Download playlist items in reverse order

Series of videos are typically uploaded to YouTube playlists in
chronological order.  By default, these videos are downloaded
latest-to-earliest; this is great for seeing the latest videos in a
series, but prevents streaming video in the order that the videos were
produced.  Add an option to download videos in reverse order,
earliest-to-latest.

Conflicts:
	youtube_dl/YoutubeDL.py
	youtube_dl/__init__.py