support a default zone TTL

this is supported in the DNS spec with the `$TTL` directive, but not supported by `dnspython` (see https://github.com/rthalley/dnspython/issues/63).

Will need to add custom handling to this in the ZFZone class, and when getting/writing zones,

Start with the ZFZone class and the code paths for getting and writing zones, then review the DNS specification for the $TTL directive and the linked dnspython issue. Done means zones can preserve and apply a default zone TTL when they are read and written.

Written by the indexing model from the issue text.