Regenerate a specific image size for one or more images #1

On an existing production site, it can be a little bit scary to regenerate all image sizes for all images because of the fear of the unknown.

If I've only changed the width / height values for one image size, it would be nice to be able to only regenerate that specific image size.

For instance, if I've changed thumbnail from 183x183 to 185x185, I could run wp media regenerate --image_size=thumbnail to solely regenerate thumbnails.