Describe the new feature or change to an existing feature you'd like to see

Normal Vimeo URLs look like: https://vimeo.com/172355597

But Vimeo also has the ability to create unlisted videos: https://vimeo.com/blog/post/share-unlisted-videos-with-private-links/

Such unlisted video URLs look like: https://vimeo.com/172355597/abcdef0123

However, the amp-vimeo component has no way to supply the unlisted secret key. It only has a data-videoid attribute.

Additional context

This issue was discovered as part of the AMP plugin for WordPress: ampproject/amp-wp#2986