Enable embedding PhET STEM education simulations in annotations

Educators like to engage students with STEM interactive simulations published by the University of Colorado's [PhET](https://phet.colorado.edu/).

Just like with [H5P](https://github.com/hypothesis/client/issues/509#issue-246080068), PhET offers embed functionality (via iframes) and so would be a similar use case, workflow, and possibly [implementation](https://github.com/hypothesis/client/issues/509#issuecomment-318413040) as H5P (or other already supported embeds, like YouTube, Vimeo, Flipgrid, etc).

To learn more about PhET, visit their [website](https://phet.colorado.edu/), [view a presentation](https://drive.google.com/file/d/12htlZr3g-AVlS6UjRGsFzChmH8dWq5RR/view), and/or reach out to a contact like [Kathy Perkins](mailto:perkinsk@colorado.edu), Director, who has had conversations with @xolotl about this feature.