generate: missing proto file options

[vishen]

We are missing the cc and python proto file options from https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/descriptor.proto#L318, we should add them to https://github.com/gunk/gunk/blob/master/generate/generate.go#L443.

[vishen]

Actually, we need to add the python_generic_services to gunk-opt. cc_generic_services and cc_enable_arenas are already in gunk-opt.

All the generic_services options are considered somewhat deprecated in favour of plugins, but we may as well continue to support them until proto finally drops the,