Add on plugin (#1412)

Enable this Caddy plugin by default. Docs will go up coredns.io for this
as well.

See https://caddyserver.com/docs/on

core/dnsserver/zdirectives.go

@@ -41,6 +41,7 @@ var Directives = []string{
 	"proxy",
 	"erratic",
 	"whoami",
+	"on",
 	"startup",
 	"shutdown",
 }