add String() method to zone

This way we can print it in nicer ways.

types/zone.go

@@ -19,3 +19,7 @@ func (z *ZoneName) UnmarshalJSON(in []byte) error {
 	*z = ZoneName(in)
 	return nil
 }
+
+func (z ZoneName) String() string {
+	return string(z)
+}