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

youtube_dl/options.py

@@ -349,6 +349,10 @@ def parseOpts(overrideArguments=None):
         '--test',
         action='store_true', dest='test', default=False,
         help=optparse.SUPPRESS_HELP)
+    downloader.add_option(
+        '--playlist-reverse',
+        action='store_true',
+        help='Download playlist videos in reverse order')
 
     workarounds = optparse.OptionGroup(parser, 'Workarounds')
     workarounds.add_option(