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

middleware/reverse/network.go

@@ -13,7 +13,6 @@ type network struct {
 	Template     string
 	TTL          uint32
 	RegexMatchIP *regexp.Regexp
-	Fallthrough  bool
 }
 
 // TODO: we might want to get rid of these regexes.