Add support for METV

[dare2]

Please follow the guide below

• You will be asked some questions and requested to provide some information, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your issue (like this: [x])
• Use the Preview tab to see what your issue will actually look like

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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.12.31. 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 Issues with outdated version will be rejected.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 2017.12.31

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the 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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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: https://www.metv.com/videos/my-favorite-martian/unaired-pilot
• Single video: https://www.metv.com/videos/mystery-science-theater-3000/the-corpse-vanishes

Note that youtube-dl does not support sites dedicated to copyright infringement. In order for site support request to be accepted all provided example URLs should not violate any copyrights.

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Description of your issue, suggested solution and other information

Please investigate supporting downloads from the metv.com site. I believe this is a newly launched service from the METV network.