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.

README.md

@@ -84,6 +84,7 @@ which means you can modify it, redistribute it or use it however you like.
 ## Video Selection:
     --playlist-start NUMBER          playlist video to start at (default is 1)
     --playlist-end NUMBER            playlist video to end at (default is last)
+    --playlist-reverse               Download playlist videos in reverse order
     --match-title REGEX              download only matching titles (regex or
                                      caseless sub-string)
     --reject-title REGEX             skip download for matching titles (regex or

youtube_dl/YoutubeDL.py

@@ -114,6 +114,7 @@ class YoutubeDL(object):
     nooverwrites:      Prevent overwriting files.
     playliststart:     Playlist item to start at.
     playlistend:       Playlist item to end at.
+    playlistreverse:   Download playlist items in reverse order.
     matchtitle:        Download only matching titles.
     rejecttitle:       Reject downloads for matching titles.
     logger:            Log messages to a logging.Logger instance.
@@ -623,8 +624,9 @@ class YoutubeDL(object):
                 self.to_screen(
                     "[%s] playlist %s: Downloading %d videos" %
                     (ie_result['extractor'], playlist, n_entries))
+            step = -1 if self.params.get('playlistreverse', False) else 1
 
-            for i, entry in enumerate(entries, 1):
+            for i, entry in enumerate(entries[::step]):
                 self.to_screen('[download] Downloading video #%s of %s' % (i, n_entries))
                 extra = {
                     'playlist': playlist,

youtube_dl/__init__.py

@@ -287,6 +287,10 @@ def parseOpts(overrideArguments=None):
         '--playlist-end',
         dest='playlistend', metavar='NUMBER', default=None, type=int,
         help='playlist video to end at (default is last)')
+    selection.add_option(
+        '--playlist-reverse',
+        action='store_true',
+        help='Download playlist videos in reverse order')
     selection.add_option('--match-title', dest='matchtitle', metavar='REGEX',help='download only matching titles (regex or caseless sub-string)')
     selection.add_option('--reject-title', dest='rejecttitle', metavar='REGEX',help='skip download for matching titles (regex or caseless sub-string)')
     selection.add_option('--max-downloads', metavar='NUMBER',
@@ -757,6 +761,7 @@ def _real_main(argv=None):
         'progress_with_newline': opts.progress_with_newline,
         'playliststart': opts.playliststart,
         'playlistend': opts.playlistend,
+        'playlistreverse': opts.playlist_reverse,
         'noplaylist': opts.noplaylist,
         'logtostderr': opts.outtmpl == '-',
         'consoletitle': opts.consoletitle,