Added a Google Protobuf mode

demo/kitchen-sink/docs/protobuf.proto

@@ -0,0 +1,16 @@
+message Point {
+  required int32 x = 1;
+  required int32 y = 2;
+  optional string label = 3;
+}
+
+message Line {
+  required Point start = 1;
+  required Point end = 2;
+  optional string label = 3;
+}
+
+message Polyline {
+  repeated Point point = 1;
+  optional string label = 2;
+}