Add EDNS Client Subnet option

[PenelopeFudd]

Describe the feature requested
We've set up a DNS server that uses the EDNS0 Client Subnet (ECS) option (RFC 7871) to return the ip address of the closest endpoint. Now we'd like to test this to see if it works quickly, correctly and efficiently.

Examples of using ECS with dig:

$ dig +short @8.8.8.8 TXT o-o.myaddr.l.google.com +subnet=1.2.3.4/24

"edns0-client-subnet 1.2.3.0/24"
"7.15.87.9"

$ dig +short @8.8.8.8 TXT o-o.myaddr.l.google.com +subnet=2001:7f1::/48

"edns0-client-subnet 2001:7f1::/48"
"7.15.86.9"

Additional context
One thing we'd like to check is to see if our server is modifying the ECS option, such that the subnet in the request is different than the subnet in the response.

Solution
Allow the input text to have @x.x.x.x/y appended to the FQDN input names, which would then be parsed as the subnet to use. Or, use CSV or TSV format, in case someone starts using @ in FQDNs.