[imgur] support hidden albums and descriptions

[PeskyPotato]

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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

Hidden albums containing mp4 (gifv) or gif files were not supported after Imgur changed their API as it relied on their public gallery JSON file location. In addition, descriptions for each file can now be written with --write-description on both hidden albums and public galleries.