Support literal IPv6 nameservers

This effectively means that `doggo @2001:DB8::53 example.org` works just
like we know it from `dig(1)`. This already works for IPv4 because
`url.Parse` accepts a literal IPv4 address, but not a literal IPv6
address (due to parsing ambiguities with strings that look like an IPv4
address I guess).

This is implemented by checking if the nameserver argument is a valid IP
if the URL validation failed. If that's the case, `ns` is returned
directly with the `[2001:DB8::53]:53` as address (from
`net.JoinHostPort` with the default DNS port) and UDP as protocol (also
the default if not specified).

internal/app/nameservers.go

@@ -61,8 +61,12 @@ func initNameserver(n string) (models.Nameserver, error) {
 	}
 	u, err := url.Parse(n)
 	if err != nil {
+		ip := net.ParseIP(n)
+		if ip == nil {
 			return ns, err
 		}
+		return ns, nil
+	}
 	switch u.Scheme {
 	case "sdns":
 		stamp, err := dnsstamps.NewServerStampFromString(n)