middleware/auto: add (#333)

Add auto-load middleware that automatically picks up zones.

Every X seconds it will scan for new zones.
Add tests and documentation.

Make 'make test' use -race.

core/coredns.go

@@ -5,7 +5,8 @@ import (
 	// plug in the server
 	_ "github.com/miekg/coredns/core/dnsserver"
 
-	// plug in the standard directives
+	// plug in the standard directives (sorted)
+	_ "github.com/miekg/coredns/middleware/auto"
 	_ "github.com/miekg/coredns/middleware/bind"
 	_ "github.com/miekg/coredns/middleware/cache"
 	_ "github.com/miekg/coredns/middleware/chaos"

core/dnsserver/directives.go

@@ -89,6 +89,7 @@ var directives = []string{
 
 	"dnssec",
 	"file",
+	"auto",
 	"secondary",
 	"etcd",
 	"kubernetes",