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.