Writing bind files with ipaddress

Hello, 

i used ipaddress to write name server configuration. And there is the reverse function very handy but 
to write a complete file you need to write a SOA record which contains a shorter string than fqdn. So
i added the function to generate these special strings for ipv4 and ipv6. 

On that way a added function to ipv6 which allows the same like supernet/subnet in ipv4.

cheers

meno