[willowtv] Add new extractor

[philipl]

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:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

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

willow.tv is a Cricket TV channel and Internet service. On the Internet,
they offer live streaming, full replays, and highlight videos.

This initial commit handles downloading of replays.

Note that replays are only accessible with an active subscription, and
youtube-dl must be passed the cookies for an active login session. That
means that it's not possible to run a test case in general, and I think
not possible at all, as I don't think there's a way to pass cookies when
running the tests.

[dstftw]

1. You must provide account credentials.
2. Read coding conventions.

[philipl]

1. You must provide account credentials.

Ah. Well, I'm afraid I'm not in a position to do that, although I can understand why you have that as a policy. I'll just keep maintaining it on my own.

Thanks.