Unqualified domain names

[pigam]

This adds a useOrigin option to zone type. It is a boolean, equals to false by default.
By default, it changes nothing to the actual result of toString.

If set to true, this change the toString behaviour to add $ORIGIN directive, and use @ as domain name for the zone itself.
writeRecord has been patched accordingly if called with @ as zone name.

This should address #17

I had the need for a RPZ zone.

[felbinger]

I also need to implement a response policy zone, can this be merged?

[Tom-Hubrecht]

A sentence or two regarding this new behaviour should be addded to the README as documentation

[pigam]

A sentence or two regarding this new behaviour should be addded to the README as documentation

I moved example.com.nix and wrote an example of a call to toString, see commit f084caf

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Then use toString to generate as a string.

{ dns }:

  zoneAsString = dns.lib.toString "example.com" (import ./example.com.nix) { inherit dns; };

If useOrigin=false, the default value, serialization will use fully qualified name.
For instance, A = [ "203.0.113.1" ]will be serialized as example.com. A 203.0.113.1.

If useOrigin=true, toString adds $ORIGIN example.com. and use @ as domain name.
A = [ "203.0.113.1" ]will be then serialized as

$ORIGIN example.com.
@ A 203.0.113.1