* Handle IPv4 "Classless in-addr.arpa delegation" RFC2317 (partial).
* Validate PTR name when in RFC2317 "Classless in-addr.arpa delegation" domains.
* Update docs
* Set CanUsePTR for Route53 and Google CloudDNS.
* BIND: Replace "/" with "_" in filenames.
* WIP
* Enable PTR records in dnsconfig.js, in BIND provider.
* Rename REVERSE() to REV().
* More accurate PTR target checking
* Document REV()
* Fix broken test