Support for multiple extraVirtualAliases

Created by: geistesk

Should fix #104 (closed) by introducing

extraVirtualAliases = {
  "single-alias@domain.foobar" = "user1@domain.foobar";
  "multi-alias@domain.foobar" = [
    "user1@domain.foobar" "user2@domain.foobar" ];
};