allow restriction of destination ip address

[exaV]

See https://github.com/serjs/socks5-server/issues/49

We no longer throw an error if we connect to IP address while a ALLOWED_DEST_FQDN is set, instead we check that the destination IP address matches the pattern. This is the simplest possible change that allows users to restrict which IP addresses can be connected to. A more elaborate change would be to introduce a new environment variable that reflects that both IP addresses and domain names can be restricted. I don't mind either solution.