Add support for subtitles fro DPlay

[TRox1972]

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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2016.04.19. If it's not read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2016.04.19

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• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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The following sections concretize particular purposed issues, you can erase any section (the contents between triple ---) not applicable to your issue

If the purpose of this issue is a site support request please provide all kinds of example URLs support for which should be included (replace following example URLs by yours):

• Single video: http://www.dplay.no/dr-ken/season-1-episode-3

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Description of your issue, suggested solution and other information

Can you add support for subtitles for the extractor DPlay?