adgnumailconverter converts old GNUMail addresses to Addresses format.
The program uses the following heuristic to split GNUMail's name field into Addresses's first/last name
fields:
1. Name contains a ",": Split it at the "," and put the first part into last name and the second part
into first name.
"Mueller, Fritz" -> "Mueller", "Fritz"
2. Name doesn't contain a ",": Split it at spaces and put the last part into last name and everything
else into first name.
"Johnny B. Goode" -> "Goode", "Johnny B."