middlewware/startup|shutdown (#604)

Add middleware by directly linking it from caddy, i.e. without any code
changes. To be fair: this does not added a ServeHTTP, but does give
some nice features in the Corefile.

core/dnsserver/zdirectives.go

@@ -34,4 +34,6 @@ var directives = []string{
 	"proxy",
 	"whoami",
 	"erratic",
+	"startup",
+	"shutdown",
 }