Add support for downloading audio-only track for on-demand TV streams

[tmthywynn8]

Though I do not know of any other use cases for this request, some blind/visually impaired users may want this option so they can download the Audio Described audio tracks only, and not waste any bandwidth on video. Currently, if one wanted to keep only the audio track, one could use the --raw option on DVF streams, but the video track would be downloaded as well.

Here's an example from the first episode of Class (PID p048h1s0) from the Akamai server, using the HLS streaming protocol:

1. Find the HLS master playlist:
   • Example: http://vod-hls-uk-live.akamaized.net/usp/auth/vod/piff_abr_full_sd_ad/c4f2ed-p04cb0dw/vf_p04cb0dw_ad04db32-bbe2-41c3-acd2-0f6d53e74284.ism.hlsv2.ism/iptv_hd_abr_v1_hls_master.m3u8?__gda__=1486468585_e2b0e6319de19f6d5625032a204e6e06
2. Find the corresponding audio stream (audio_1=128000). You could find the audio track(s) by opening the playlist and modifying the relative URLs, or If you use a Regexp, you could:
   • Find: (/(vf_.+.ism.hlsv2).ism/)iptv_hd_abr_v1_hls_master
   • Replace: \1\2-audio_1=128000
   • Result: http://vod-hls-uk-live.akamaized.net/usp/auth/vod/piff_abr_full_sd_ad/c4f2ed-p04cb0dw/vf_p04cb0dw_ad04db32-bbe2-41c3-acd2-0f6d53e74284.ism.hlsv2.ism/vf_p04cb0dw_ad04db32-bbe2-41c3-acd2-0f6d53e74284.ism.hlsv2-audio_1=128000.m3u8?__gda__=1486468585_e2b0e6319de19f6d5625032a204e6e06
3. Download all fragments and combine to M4A file.

I don't know how to do this with DASH, but I assume since the code is already in GiP to download the audio and video tracks into separate files already, an option could be created to download just the audio track for the DASH and HLS protocols?

[dinkypumpkin]

Post feature requests at: https://squarepenguin.co.uk/forums/forum-7.html