Add field type FT_IGNORE to generator.

This allows ignoring fields that are unnecessary or too large for an
embedded system using nanopb, while allowing them to remain in the .proto
for other platforms.

Update issue 51
Status: FixedInGit

generator/nanopb.proto

@@ -11,6 +11,7 @@ enum FieldType {
     FT_DEFAULT = 0; // Automatically decide field type, generate static field if possible.
     FT_CALLBACK = 1; // Always generate a callback field.
     FT_STATIC = 2; // Generate a static field or raise an exception if not possible.
+    FT_IGNORE = 3; // Ignore the field completely.
 }
 
 message NanoPBOptions {