Allow defining field type in .proto.

Update issue 30
Status: FixedInGit

tests/options.expected

@@ -1,3 +1,5 @@
 char filesize[20];
 char msgsize[30];
 char fieldsize[40];
+pb_callback_t int32_callback;
+

tests/options.proto

@@ -25,4 +25,9 @@ message Message3
     required string fieldsize = 1 [(nanopb).max_size = 40];
 }
 
+// Forced callback field
+message Message4
+{
+    required int32 int32_callback = 1 [(nanopb).type = FT_CALLBACK];
+}