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/dnsserver/directives.go

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