Support extended DNS (EDNS0) #100

We should add support for extended DNS (EDNS0) to support DNS messages larger than 512 bytes over UDP (#12) as an alternative to requiring transport via TCP (#47).

See https://tools.ietf.org/html/rfc6891 (and older https://tools.ietf.org/html/rfc2671)

I'm not working on this at the moment, but figured it's worth posting this here anyway. In case anybody feels like picking this up, PRs would be much appreciated +1