Add middleware/erratic (#471)

This middleware allows playing with responses. Only one type is
implemented: it allows you to drop queries. I.e. withhold the response
from the client.

core/coredns.go

@@ -11,6 +11,7 @@ import (
 	_ "github.com/miekg/coredns/middleware/cache"
 	_ "github.com/miekg/coredns/middleware/chaos"
 	_ "github.com/miekg/coredns/middleware/dnssec"
+	_ "github.com/miekg/coredns/middleware/erratic"
 	_ "github.com/miekg/coredns/middleware/errors"
 	_ "github.com/miekg/coredns/middleware/etcd"
 	_ "github.com/miekg/coredns/middleware/file"

core/dnsserver/directives.go

@@ -96,4 +96,5 @@ var directives = []string{
 	"proxy",
 	"httpproxy",
 	"whoami",
+	"erratic",
 }