Would be nice to have support for objects which can't be edited once added (either just by disabling click editing or disabling future .update calls).

Boolean could be set either on the GeoJSON properties or passed directly to .set.

I started a quick & dirty version at https://github.com/drewbo/gl-draw/tree/permanent for my own purposes but can work on it further (docs + tests) if this is a desired feature.