Add Udp Support

Official RFC description of UDP:

>   Another quote of the day service is defined as a datagram based
>   application on UDP.  A server listens for UDP datagrams on UDP port
>   17.  When a datagram is received, an answering datagram is sent
>   containing a quote (the data in the received datagram is ignored).

## Valid UDP interface:

- [ ] Listen for UDP on port
- [ ] When datagram is received, serve quote
- [ ] Ignore all data the client sends
- [ ] TCP should still work

## Try it out

1. Start server `go run qotd.go wisdom.txt`
2. Connect with netcat `echo -n " " | nc -4u -w1 localhost 3333`
3. You should see a quote!