i.e. to enable cite2c, you'd use
`jupyter nbextension enable cite2c/main`.
Open questions:
1. Should these commands accept multiple extension names in one call?
nbextension install currently doesn't, so there's a symmetry argument,
but it would be easy to handle more than one if necessary.
2. Especially for disable, should it give an error if you try to disable
something that's not enabled? Currently it will just pass silently.
Enable could also fail if it's already enabled, but that seems less
useful.