graphb.ArgumentCustomType is added.
See source here: https://github.com/udacity/graphb/blob/50d1b4526eb80692254687bfbffb1f41370a9622/argument.go#L76
See example here:
https://github.com/udacity/graphb/blob/50d1b4526eb80692254687bfbffb1f41370a9622/query_test.go#L44
The ending , is removed from all generated strings.