Add IPv6 support

[friedrich]

[Masaq-]

Implemented in #16

[Masaq-]

For documentation, how to drop duplicate ICMPv6 echo reply:

ip6tables -A OUTPUT -p ipv6-icmp -m icmp6 --icmpv6-type 129 -m string --string "hanc" --algo bm --from 48 --to 52 -j DROP