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/dnsserver/directives.go

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