Handle unsupported extension field types more gracefully.

Previously the generator would stop with NotImplementedException as
soon as a required or repeated extension field is found. New behaviour
is to just ignore the unsupported field and note that in a comment
in the generated file.

Furthermore, allow skipping of extension fields using the generator
option (nanopb).type = FT_IGNORE.

Update issue 83
Status: FixedInGit

tests/extensions.proto

@@ -7,6 +7,8 @@ extend AllTypes {
 message ExtensionMessage {
     extend AllTypes {
         optional ExtensionMessage AllTypes_extensionfield2 = 254;
+	required ExtensionMessage AllTypes_extensionfield3 = 253;
+	repeated ExtensionMessage AllTypes_extensionfield4 = 252;
     }
     
     required string test1 = 1;