Make PTR more magical (#148)

* Initial code and tests

docs/_functions/domain/PTR.md

@@ -0,0 +1,40 @@
+---
+name: PTR
+parameters:
+  - name
+  - target
+  - modifiers...
+---
+
+PTR adds a PTR record to the domain.
+
+The name should be the relative label for the domain, or may be a FQDN that ends with `.`.
+
+* If the name is a valid IP address, DNSControl will *magically* replace it with a string that is appropriate for the domain. That is, if the domain ends with `in-addr.arpa` it will generate the IPv4-style reverse name; if the domain ends with `ipv6.arpa` it will generate the IPv6-style reverse name.  DNSControl will truncate it as appropriate for the netmask.
+* If the name ends with `in-addr.arpa.` or `ipv6.arpa.` (not the `.` at the end), DNSControl will truncate it as appropriate for the domain. If the FQDN does not fit within the domain, this will raise an error.
+
+Target should be a string representing the FQDN of a host.  Like all FQDNs in DNSControl, it must end with a `.`.
+
+{% include startExample.html %}
+{% highlight js %}
+D(REV('1.2.3.0/24'), REGISTRAR, DnsProvider(BIND),
+  PTR('1', 'foo.example.com.'),
+  PTR('2', 'bar.example.com.'),
+  PTR('3', 'baz.example.com.'),
+  // If the first parameter is a valid IP address, DNSControl will generate the correct name:
+  PTR('1.2.3.10', 'ten.example.com.'),    // '10'
+);
+
+D(REV('2001:db8:302::/48'), REGISTRAR, DnsProvider(BIND),
+  PTR('1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0', 'foo.example.com.'),  // 2001:db8:302::1
+  // If the first parameter is a valid IP address, DNSControl will generate the correct name:
+  PTR('2001:db8:302::2', 'two.example.com.'),                          // '2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0'
+  PTR('2001:db8:302::3', 'three.example.com.'),                        // '3.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0'
+);
+
+{%endhighlight%}
+{% include endExample.html %}
+
+In the future we plan on adding a flag to `A()` which will insert
+the correct PTR() record if the approprate `.arpa` domain has been
+defined.