Support gRPC Server Options

[docktermj]

Currently, the gRPC Server Options is a hard-coded, empty list,

observe/cmd/root.go

Lines 82 to 92 in 6e6cf92

	// TODO: Support various gRPC server options.
	serverOptions := []grpc.ServerOption{}
	// Create and run gRPC server.
	observer := &observer.SimpleObserver{
	AvoidServing: viper.GetBool(avoidServe.Arg),
	Port: viper.GetInt(option.ObserverGrpcPort.Arg),
	ServerOptions: serverOptions,
	}

Feature request

Using viper, receive command-line and environment variable values that can be used to make grpc.ServerOptions.