Embed additional props for accessibility and others

[Slooowpoke]

When using the component, it would be nice to be able to pass down props onto the iframe. I couldn't find anything in the docs for it, but perhaps I've missed something?

Currently the component wraps an iframe in the div and props like aria-label or title are attached to the parent div instead of the iframe. Perhaps having something like iframeProps: {} is too verbose and not in keeping with ThemeUI but I can't think of anything that would work better.

An alternative could be for me to create a custom component with the nessecary props and I'm happy to do it if that's the recommended guidelines.

Additional context
It's mostly for some accessibility reasons but I'm sure there are other cases where it would be helpful.

[lachlanjc]

To add clarity, I believe this issue is referring to the Embed component specifically

[lachlanjc]

Related: #706