Add new extractor for Mildom.

[rli-029]

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• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
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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

This Pull Request adds a new extractor for Mildom (mildom.com). Mildom is a relatively new Japanese live-streaming and video hosting platform with a focus on games. This PR adds an extractor for downloading videos from the site.