[Playvids] Add new extractor

[AXDOOMER]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

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Before submitting a pull request make sure you have:

• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This PR adds a new extractor for playvids.com

Many URLs which could trigger edge cases were tested and no bug was found.

[dstftw]

Read coding conventions.

[AXDOOMER]

@dstftw I did read them before coding. I also ran flake8 after coding to make sure everything was fine. Can you pinpoint what's wrong?

[cypheron]

I guess because of the huge backlog @dstftw doesn't actually check any code, he writes that "read the conventions" nonsense over all PR's without ever mentioning a specific thing. Hope that @dstftw can get off his horse and does some work, otherwise I don't know if my excitement about ytdl being back was really appropriate.