Add support to generate from .proto only

[moonlight2596]

This is an amazing tool. But can we have an option to read just the input from a proto file, including rpc methods, and generate a service based on that? Does the service interface have to be in a .go file, because a service definition is just as complete in a protobuf file. Truss does this, but truss does not have as many features as this tool has.