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

directives_generate.go

@@ -37,6 +37,9 @@ func main() {
 		priority, err := strconv.Atoi(items[0])
 		fatalIfErr(err)
 
+		if v, ok := md[priority]; ok {
+			log.Fatalf("Duplicate priority '%d', slot already taken by %q", priority, v)
+		}
 		md[priority] = items[1]
 		mi[items[1]] = middlewarePath + items[2] // Default, unless overriden by 3rd arg