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/_providers/bind.md

@@ -18,11 +18,13 @@ you can specify a `directory` where the provider will look for and create zone f
 {% highlight json %}
 {
   "bind": {
-    "directory": "myzones"
+    "directory": "myzones",
+    "filenameformat": "%U.zone"      << The default
   }
 }
 {% endhighlight %}
 
+
 The BIND accepts some optional metadata via your DNS config when you create the provider:
 
 In this example we set the default SOA settings and NS records.
@@ -46,15 +48,55 @@ var BIND = NewDnsProvider('bind', 'BIND', {
 })
 {% endhighlight %}
 
+# filenameformat
+
+The `filenameformat` parameter specifies the file name to be used when
+writing the zone file. The default is acceptable in most cases: the
+name as specified in the `D()` function, plus ".zone".
+
+The filenameformat is a string with a few printf-like `%` verbs:
+
+  * `%U`  the domain name as specified in `D()`
+  * `%D`  the domain name without any split horizon tag
+  * `%T`  the split horizon tag, or "", see `D()`
+  * `%?x` this returns `x` if the split horizon tag is non-null, otherwise nothing. `x` can be any printable.
+  * `%%`  `%`
+  * ordinary characters (not `%`) are copied unchanged to the output stream
+  * FYI: format strings must not end with an incomplete `%` or `%?`
+  * FYI: `/` or other filesystem separators result in undefined behavior
+
+Typical values:
+
+  * `%U.zone` (The default)
+    * `example.com.zone` or `example.com!tag.zone`
+  * `%T%*U%D.zone`  (optional tag and `_` + domain + `.zone`)
+    * `tag_example.com.zone` or `example.com.zone`
+  * `db_%T%?_%D`
+    * `db_inside_example.com` or `db_example.com`
+  * `db_%D`
+    * `db_example.com`
+
+The last example will generate the same name for both
+`D("example.tld!inside")` and `D("example.tld!outside")`.  This
+assumes two BIND providers are configured in `creds.json`, eacch with
+a different `directory` setting. Otherwise `dnscontrol` will write
+both domains to the same file, flapping between the two back and
+forth.
+
 # FYI: get-zones
 
-When used with "get-zones", specifying "all" scans the directory for
+The dnscontrol `get-zones all` subcommand scans the directory for
 any files named `*.zone` and assumes they are zone files.
 
 ```
 dnscontrol get-zones --format=nameonly - BIND all
 ```
 
+If `filenameformat` is defined, `dnscontrol` makes an guess at which
+filenames are zones but doesn't try to hard to get it right, which is
+mathematically impossible in all cases.  Feel free to file an issue if
+your format string doesn't work. I love a challenge!
+
 # FYI: SOA Records
 
 DNSControl assumes that SOA records are managed by the provider.  Most