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/YoutubeDL.py

@@ -124,6 +124,7 @@ 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.
@@ -670,6 +671,9 @@ class YoutubeDL(object):
                     "[%s] playlist %s: Downloading %d videos" %
                     (ie_result['extractor'], playlist, n_entries))
 
+            if self.params.get('playlistreverse', False):
+                entries = entries[::-1]
+
             for i, entry in enumerate(entries, 1):
                 self.to_screen('[download] Downloading video #%s of %s' % (i, n_entries))
                 extra = {