[feature] heartbeat, [niconico] fix: download video

[insaneracist]

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:

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This is based on unmerged patches by
@Gorfiend unqueued/youtube-dl-prs@8499534
@nekojun unqueued/youtube-dl-prs@5a6f5f9
I used a thread that sleeps rather than creating a timer thread on each heartbeat. Btw, @unqueued did great work making that pull request repo.

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Fixes #72
This adds a Heartbeat feature that makes requests at some interval, while downloading. Some video sites require this, or they cancel the download.