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.