NEW FEATURE: Support Split Horizon DNS (#1034)

* Implement main feature
* BIND: Permit printf-like file name formats
* BIND: Make filenameformat work forwards and backwards.
* Fix extrator test cases

docs/_functions/global/D.md

@@ -42,6 +42,56 @@ D("example.com", REGISTRAR, DnsProvider(r53),
   CNAME("test", "foo.example2.com."),
   GOOGLE_APPS_DOMAIN_MX
 );
-
 {%endhighlight%}
 {% include endExample.html %}
+
+
+# Split Horizon DNS
+
+DNSControl supports Split Horizon DNS. Simply
+define the domain two or more times, each with
+their own unique parameters.
+
+To differentiate the different domains, specify the domains as
+`domain.tld!tag`, such as `example.com!inside` and
+`example.com!outside`.
+
+{% include startExample.html %}
+{% highlight js %}
+var REG = NewRegistrar("Third-Party", "NONE");
+var DNS_INSIDE = NewDnsProvider("Cloudflare", "CLOUDFLAREAPI");
+var DNS_OUTSIDE = NewDnsProvider("bind", "BIND");
+
+D("example.com!inside", REG, DnsProvider(DNS_INSIDE),
+  A("www", "10.10.10.10")
+);
+
+D("example.com!outside", REG, DnsProvider(DNS_OUTSIDE),
+  A("www", "20.20.20.20")
+);
+
+D_EXTEND("example.com!inside",
+  A("internal", "10.99.99.99")
+);
+{%endhighlight%}
+{% include endExample.html %}
+
+A domain name without a `!` is assigned a tag that is the empty
+string. For example, `example.com` and `example.com!` are equivalent.
+However, we strongly recommend against using the empty tag, as it
+risks creating confusion.  In other words, if you have `domain.tld`
+and `domain.tld!external` you now require humans to remember that
+`domain.tld` is the external one.  I mean... the internal one.  You
+may have noticed this mistake, but will your coworkers?  Will you in
+six months? You get the idea.
+
+DNSControl command line flag `--domains` is an exact match.  If you
+define domains `example.com!george` and `example.com!john` then:
+
+* `--domains=example.com` will not match either domain.
+* `--domains='example.com!george'` will match only match the first.
+* `--domains='example.com!george',example.com!john` will match both.
+
+NOTE: The quotes are required if your shell treats `!` as a special
+character, which is probably does.  If you see an error that mentions
+`event not found` you probably forgot the quotes.