Fix stderr printing functions (#894)

* Fix stderr printing functions

Follow-up of #822

* Errors go to stderr as per POSIX

* use PrintErrf() instead of extra call to Sprintf()

* Error messages should always be printed to os.Stderr.

* add test case for Print* redirection

Thanks: @bukowa for the patch.

README.md

@@ -177,7 +177,7 @@ var rootCmd = &cobra.Command{
 
 func Execute() {
   if err := rootCmd.Execute(); err != nil {
-    fmt.Println(err)
+    fmt.Fprintln(os.Stderr, err)
     os.Exit(1)
   }
 }