Domainer registration

This fixes issue #618 
Looks like Dan's using a `string` key, so you would call `paddingException(null, key)` to remove the exception associated with `key`. I think this is less intuitive because there are now two identifying features per exception (the value itself and the key). Also, the suggested key is an underscored-protected field (`_plottableID`) which won't show up in the .d.ts file.