richannel Add new extractor

[mcjevans]

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• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
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• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

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I want to add support for the Royal Institute Christmas Lectures as described in Issue 4739. The extractor locates and passes along the location of an MP4 file hosted on the site itself that the HTML5 player displays.

[dstftw]

Carefully read new extractor tutorial and youtube-dl coding conventions and fix your code to follow the conventions.

[mcjevans]

As near as I can tell, it's only failing because it's running out of time. And it's running out of time because it takes so long to run through every test. When the tutorial says to "Add an import in youtube_dl/extractor/extractors.py" did it really mean to only put my import in the extractors.py file I add?

[mpenkov]

Just checking up on this PR. Travis CI builds are failing, but there is nothing in the logs at all - they are empty. Perhaps the builds are (were?) failing because of a problem in Travis CI, not a problem in the PR? Could we try a rebuild to confirm whether that's the case?

The PR looks good otherwise.