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

[tansy]

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Description of your pull request and other information

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

[dirkf]

Trivial, but you should add a test for the changed URL pattern, which you promised to do in the PR checklist IIRC.

If you use one of the existing test videos, it can just be an only_matching test:

        {
            'url': 'https://streamable.com/t/dnd1',
            'only_matching': True,
        }

[tansy]

I did test it before pushing, just didn't get thoroughly what you meant by 'test'.

I hope that's what you mean. If not, I don't know.

BTW, this video
doesn't work any more.

[MahmoudMabrok]

@tansy tests, mean to add a code to test the new code added, there are (unit testing, integration testing).

This is example