[rtp] Add support for playlists

[accorreia123]

It seems not be possible to download from rtp.pt, the videos on one page, as we can do in a youtube playlist.

I tried and get this:

D:\Downloads>youtube-dl -f 22/17/18 --verbose -o ./%(playlist_title)s/%(playlist_index)s-%(title)s.%(ext)s http://www.rtp.pt/play/p831/a-quimica-das-coisas
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['-f', '22/17/18', '--verbose', '-o', './%(playlist_title)s/%(playlist_index)s-%(title)s.%(ext)s', 'http://www.rtp.pt/play/p831/a-quimica-das-coisas']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2015.02.11
[debug] Python version 2.7.8 - Windows-8-6.2.9200
[debug] exe versions: rtmpdump 2.3
[debug] Proxy map: {}
[RTP] a-quimica-das-coisas: Downloading webpage
ERROR: requested format not available
Traceback (most recent call last):
File "youtube_dl\YoutubeDL.pyo", line 644, in extract_info
File "youtube_dl\YoutubeDL.pyo", line 690, in process_ie_result
File "youtube_dl\YoutubeDL.pyo", line 1137, in process_video_result
ExtractorError: requested format not available