Provide option or IPAM method to enable SLAAC

#434 has been a rather annoying change for me. Since I don't have the luxury of a stable IPv6 prefix, I'm relying on SLAAC, fixed MAC addresses along with MACVLAN to get predictable addresses in my containers.
While I can add `--sysctl net.ipv6.conf.eth0.autoconf=1` for most containers, this relies on consistent naming and gets very verbose the moment multiple networks are used.
I'd love to have a way to enable SLAAC for networks again. Either through an option or maybe a dedicated IPAM method to make it very explicit.