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;