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/config.go

@@ -17,6 +17,10 @@ type Config struct {
 	// The port to listen on.
 	Port string
 
+	// Root points to a base directory we we find user defined "things".
+	// First consumer is the file middleware to looks for zone files in this place.
+	Root string
+
 	// Middleware stack.
 	Middleware []middleware.Middleware