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",
 }

core/plugin/zplugin.go

@@ -35,5 +35,6 @@ import (
 	_ "github.com/coredns/coredns/plugin/tls"
 	_ "github.com/coredns/coredns/plugin/trace"
 	_ "github.com/coredns/coredns/plugin/whoami"
+	_ "github.com/mholt/caddy/onevent"
 	_ "github.com/mholt/caddy/startupshutdown"
 )