* Integration tests: PTR tests should only happen if CanUsePTR
* Integration tests: SRE tests should only happen if CanUseSRV
* CanUseSRV should be validated early (in pkg/normalize/validate.go)
* CLOUDFLARE does not support SRV. Check for this during validation
* GCLOUD CanUsesRV (mostly by accident, but whatever works)
* Add support for SRV records for NAMEDOTCOM and ROUTE53.
* Improve docs
* Rename RR() to ToRR().
* Rename RecordConfig Priority to MxPreference (affects json IR data)
* 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
