middleware/root: add it (#330)

This PR adds the *root* middleware that specifies a path where
all zone file (the *file* middleware is the only consumer now) can
be found. It works the same as in Caddy.

Documentation can be found in the README.md of the middleware.

Fixes #307

core/dnsserver/directives.go

@@ -73,6 +73,7 @@ func RegisterDevDirective(name, before string) {
 // (after) them during a request, but they must not
 // care what middleware above them are doing.
 var directives = []string{
+	"root",
 	"bind",
 	"health",
 	"pprof",