YoutubePlaylistIE: get *all* videos

For that, we add parameter safeSearch=none that asks youtube not filter
results before sending them to us.

Note: this parameter could be added to YoutubeSearchIE and YoutubeUserIE
as well, but I don't know what would be the impact in term of unwanted
results. Maybe expose that as a parameter? For a playlist it's different
since the user chose what she put in the playlist.

youtube_dl/InfoExtractors.py

@@ -1544,7 +1544,7 @@ class YoutubePlaylistIE(InfoExtractor):
                      |
                         ((?:PL|EC|UU)[0-9A-Za-z-_]{10,})
                      )"""
-    _TEMPLATE_URL = 'https://gdata.youtube.com/feeds/api/playlists/%s?max-results=%i&start-index=%i&v=2&alt=json'
+    _TEMPLATE_URL = 'https://gdata.youtube.com/feeds/api/playlists/%s?max-results=%i&start-index=%i&v=2&alt=json&safeSearch=none'
     _MAX_RESULTS = 50
     IE_NAME = u'youtube:playlist'