Document fallthrough and fix rewrite (#537)

* Document fallthrough and fix *reverse*

While documenting the fallthrough behavior and testing it I noticed
the did not properly work. This PR does a tiny bit too much as it

- Documents fallthrough
- Fixes fallthrough in reverse
- Makes directives_generate complain on duplicate priorities
- Moved reverse *before* file in middleware.cfg
- Add a test that tests the reverse fallthrough behavior with a file
  backend

Fixes #515

* ....and fix the tests

core/dnsserver/zdirectives.go

@@ -23,12 +23,12 @@ var directives = []string{
 	"rewrite",
 	"loadbalance",
 	"dnssec",
+	"reverse",
 	"file",
 	"auto",
 	"secondary",
 	"etcd",
 	"kubernetes",
-	"reverse",
 	"proxy",
 	"whoami",
 	"erratic",