[Instagram] Add new [:tag] extractor, refactor [:user]

[jhwgh1968]

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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 adds an extractor for Instagram videos. It downloads all videos that match a given hashtag as a playlist. It is heavily based on the Instagram user extractor.

I did consider refactoring common methods instead of copy-pasting, but decided it was more important to avoid touching the other code. This way is more difficult to maintain, but I hope is easier to review and avoids the risk of regressions in the other extractors.

If you would prefer a refactor -- either now or in a separate PR -- I would be happy to do it.

[dstftw]

No copy pasting here.

[jhwgh1968]

@dstftw, I have refactored the utility functions out as you requested.

[jhwgh1968]

I have fixed two of the issues @dstftw, but I need clarification on the third one. Where is the repetition?

[jhwgh1968]

I have reworked the classes as you requested, @dstftw. I am pretty sure there is nothing left to repeat now.

[jhwgh1968]

Sorry for the extra changes, @dstftw. I did not revert all of my first attempt at the code which did not use a superclass before, and I should have.

I have put that block back the way it originally was, as you requested. However, I also added a couple of comments, because it was not obvious to me at first why it had to be a class member.

I think your other comments should be addressed also.

[jhwgh1968]

Ping @dstftw, I think I have fixed everything. Is there anything else I need to do?