Add streamable.com/t/<VID> url support

[tansy]

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Before submitting a pull request make sure you have:

• [x ] Searched the bugtracker for similar pull requests
• [x ] Read adding new extractor tutorial
• [x ] Read youtube-dl coding conventions and adjusted the code to meet them
• [x ] Covered the code with tests (note that PRs without tests will be REJECTED)
• [x ] 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:

• [x ] I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Bug fix
• [x ] Improvement
• [x ] New extractor
• New feature

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Description of your pull request and other information

Add `streamable/t/' url support. It is analogous to youtube/embed.