Allowing to register models for data extracting but not for translating

[adrianbuskiewicz]

I'm asking if you would like to consider allowing devs to register the models just for extracting data to .po files with makemessages command, but not using the VinaigretteDescriptor to translate them? I had a problem when I needed to use a second library (because vinaigrette doesn't support field lookups), and two descriptors were having compatibility problems.