Add support for 4d.rtvslo.si archive

[novadeviator]

I have:

• I've verified and I assure that I'm running youtube-dl 2018.02.11
• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Site support request (request for adding support for a new site)

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example URLs:

• Single video: http://4d.rtvslo.si/arhiv/arsov-art-atelje/174514670
• Single video: http://4d.rtvslo.si/arhiv/olimpijske-igre-2018-pyeongchang-oi2018/174519820
• Single video: http://4d.rtvslo.si/arhiv/1968/174519493
• Singel video: http://4d.rtvslo.si/arhiv/study-abroad/174518718

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Description of your issue, suggested solution and other information

Please add support for the archive of shows on slovenian national radio-television's online portal. They provide actual archive video clips from TV but also radio programs which also take form of a video - audio with a still image.

[franga2000]

I've done some reverse-engineering and it looks like it's standard HLS with some trivial DRM scheme slapped on top.
Here are my notes, which should be enough to implement this: https://gist.github.com/franga2000/688d6062b3006c3703d0bfb2d7dd626d#file-rtv_4d_download_spec-md

I can't spare the time at the moment, but am happy to implement it once I'm free (unless someone beats me to it).

If anyone comes across this more than a month from now and I haven't made a PR yet, please ping me because I probably just forgot 😅.

Any pointers from the maintainers in terms of the convention for handling M3U8 URLs are also appreciated - I imagine there's some shared function every HLS-based extractor calls since it's so common?