Allow list-groups in panels, make panel-body optional

Bootstrap example is [here](http://getbootstrap.com/components/#panels-list-group)

I haven't been able to actually see the docs build result. How do I do it once I've run `lein with-profile docs cljsbuild`?
Nice. Can you add this option to the schema above?

`(s/optional-key :list-group) t/Renderable`

should do the trick.