Ability to caption image

Please add an ability to caption images.

I know that I can get the appearance of an image caption with custom css, but having an actual caption for the image would be very helpful, especially, when moving images so you don’t have to select the two rems together.

Alternatively this could be done by being able to group two Rems (the one with the image and the one with the caption) together, so they stay together when one of the rems is moved or cut and pasted.