Collect more metadata when saving

The [JSCM format](https://github.com/njsmith/json-cm/blob/master/json-cm-spec.md) has various metadata besides just name: license, description, credits, whether or not the colormap is red-green colorblind friendly, etc. We should provide a UI for the user to enter this when saving.

Maybe a pop-up window that just has a set of fields to fill in?