Make kingping write to stdout

The pros and cons are debatable, but this brings IPMI exporter in line
with most of the other Prometheus tools, so hooray for consistency.

Fixes #97

main.go

@@ -96,6 +96,7 @@ func updateConfiguration(w http.ResponseWriter, r *http.Request) {
 func main() {
 	promlogConfig := &promlog.Config{}
 	flag.AddFlags(kingpin.CommandLine, promlogConfig)
+	kingpin.CommandLine.UsageWriter(os.Stdout)
 	kingpin.HelpFlag.Short('h')
 	kingpin.Version(version.Print("ipmi_exporter"))
 	kingpin.Parse()