Add generator option for packed structs.

Usage is:
message Foo
{
   option (nanopb_msgopt).packed_struct = true;
   ...
}

Valid also in file scope.

Update issue 49
Status: FixedInGit

tests/options.expected

@@ -4,3 +4,4 @@ char fieldsize\[40\];
 pb_callback_t int32_callback;
 \sEnumValue1 = 1
 Message5_EnumValue1
+} pb_packed my_packed_struct;

tests/options.proto

@@ -54,3 +54,11 @@ message Message5
     }
     required Enum2 field = 1 [default = EnumValue1];
 }
+
+// Packed structure
+message my_packed_struct
+{
+    option (nanopb_msgopt).packed_struct = true;
+    optional int32 myfield = 1;
+}
+