Allow to configure fritzctl log level (default to "warn" now)

The default log level used to be "info" which seems unnecessary high and
leads to "Login sucessfull" messages all over the place. This change
sets the default level to "warn", adding a flag to make it configurable.

README.md

@@ -11,6 +11,7 @@ Export information about your smarthome devices (switches, powermeters, thermost
 ```
 Usage of ./fritzbox_smarthome_exporter:
   -cert="": Path to the FRITZ!Box certificate.
+  -loglevel="warn": Logging verbosity (debug, info, warn or error)
   -noverify=false: Omit TLS verification of the FRITZ!Box certificate.
   -password="": FRITZ!Box password.
   -url="https://fritz.box": FRITZ!Box URL.

main.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/bpicode/fritzctl/fritz"
+	fritzctllogger "github.com/bpicode/fritzctl/logger"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -50,11 +51,18 @@ var (
 	urlString       = flag.String("url", "https://fritz.box", "FRITZ!Box URL.")
 	noVerify        = flag.Bool("noverify", false, "Omit TLS verification of the FRITZ!Box certificate.")
 	certificatePath = flag.String("cert", "", "Path to the FRITZ!Box certificate.")
+	loglevel        = flag.String("loglevel", "warn", "Logging verbosity (debug, info, warn, error or none)")
 )
 
 func validateFlags() {
 	var err error
 	flag.Parse()
+
+	l := &fritzctllogger.Level{}
+	if err := l.Set(*loglevel); err != nil {
+		log.Fatalln(err)
+	}
+
 	fbURL, err = url.Parse(*urlString)
 	if err != nil {
 		log.Fatalln(err)