Support "newstyle" Youtube playlist IDs.

Many playlists reported by Youtube now have the form like in:

    http://www.youtube.com/playlist?list=PL520044A3524F5E5D

where `PL` is prefixed to what youtube-dl used to use as playlist IDs. So,
while matching it, we adapt the regular expression so as to discard the `PL`
and only get the `520044A3524F5E5D` in the case of the playlist of the
example.

youtube-dl

@@ -2458,7 +2458,7 @@ class YahooSearchIE(InfoExtractor):
 class YoutubePlaylistIE(InfoExtractor):
 	"""Information Extractor for YouTube playlists."""
 
-	_VALID_URL = r'(?:https?://)?(?:\w+\.)?youtube\.com/(?:(?:view_play_list|my_playlists|artist|playlist)\?.*?(p|a|list)=|user/.*?/user/|p/|user/.*?#[pg]/c/)([0-9A-Za-z]+)(?:/.*?/([0-9A-Za-z_-]+))?.*'
+	_VALID_URL = r'(?:https?://)?(?:\w+\.)?youtube\.com/(?:(?:view_play_list|my_playlists|artist|playlist)\?.*?(p|a|list)=|user/.*?/user/|p/|user/.*?#[pg]/c/)(?:PL)?([0-9A-Za-z]+)(?:/.*?/([0-9A-Za-z_-]+))?.*'
 	_TEMPLATE_URL = 'http://www.youtube.com/%s?%s=%s&page=%s&gl=US&hl=en'
 	_VIDEO_INDICATOR = r'/watch\?v=(.+?)&'
 	_MORE_PAGES_INDICATOR = r'(?m)>\s*Next\s*</a>'