kubernetes: Add zone filtering to fallthrough (#1353)

* Add zone filtering to fallthrough

* Doh. gofmt

* Update documentation

plugin/plugin.go

@@ -83,6 +83,21 @@ func NextOrFailure(name string, next Handler, ctx context.Context, w dns.Respons
 	return dns.RcodeServerFailure, Error(name, errors.New("no next plugin found"))
 }
 
+// Fallthrough handles the fallthrough logic used in plugins that support it
+func Fallthrough(ftzones *[]string, qname string) bool {
+	if ftzones == nil {
+		return false
+	}
+	if len(*ftzones) == 0 {
+		return true
+	}
+	zone := Zones(*ftzones).Matches(qname)
+	if zone != "" {
+		return true
+	}
+	return false
+}
+
 // ClientWrite returns true if the response has been written to the client.
 // Each plugin to adhire to this protocol.
 func ClientWrite(rcode int) bool {