[GamerDVR] Add new extractor

[xenova]

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• 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
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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

Around 2 weeks ago I posted an issue regarding the decoding of the URL for GamerDVR.com (which was using the fallback extractor). The bug which I propose to solve was explained there.

Since the issue wasn't resolved, I decided to add support for a new site. Thanks, and I hope to see this in the next patch!
Examples: (Due to the nature of the URLs, it may be the case that the URLs presented below may not be the exact same as when another user runs the download command)

• https://gamerdvr.com/gamer/dacrash96lfc/video/83201118 correctly downloads: url
• https://gamerdvr.com/gamer/jedimfp/video/82181843 correctly downloads: url