Bug: grpc code generation is not gated by a flag

[tamird]

The default implementation requires a flag --go_out=plugins=grpc to generate services (see https://github.com/grpc/grpc-common/tree/master/go#optional---rebuilding-the-generated-code), but gogoproto always generates them, which is sometimes undesired (e.g. when a project is generating custom code for rpc that isn't grpc)

[awalterschulze]

Ug :(
Good point.
I didn't think anyone would notice, I mean grpc is so awesome, why wouldn't you want it.
But I think this breaks my own rule.
This is definitely a bug.

[awalterschulze]

fixed