Add roundrobin middleware

This middleware allows you to round robin a/aaaa records in a reply and
maybe more in the future (i.e.) sort a packet?

core/directives.go

@@ -51,9 +51,10 @@ var directiveOrder = []directive{
 	{"shutdown", setup.Shutdown},
 
 	// Directives that inject handlers (middleware)
+	{"prometheus", setup.Prometheus},
+	{"loadbalance", setup.Loadbalance},
 	{"log", setup.Log},
 	{"errors", setup.Errors},
-	{"prometheus", setup.Prometheus},
 	{"rewrite", setup.Rewrite},
 	{"file", setup.File},
 	{"proxy", setup.Proxy},

core/setup/loadbalance.go

@@ -0,0 +1,19 @@
+package setup
+
+import (
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/middleware/loadbalance"
+)
+
+// Root sets up the root file path of the server.
+func Loadbalance(c *Controller) (middleware.Middleware, error) {
+	for c.Next() {
+		// and choosing the correct balancer
+		// TODO(miek): block and option parsing
+	}
+	return func(next middleware.Handler) middleware.Handler {
+		return loadbalance.RoundRobin{Next: next}
+	}, nil
+
+	return nil, nil
+}