issue #5: implement stderr command

internal/cmd/demo/stderr.go

@@ -0,0 +1,27 @@
+package demo
+
+import (
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+// Stderr returns a demo cobra.Command to write to the stderr.
+//
+// 	$ go run main.go stderr [message]
+// 	$ go run main.go stderr [message] >/dev/null
+// 	$ go run main.go stderr [message] 2>/dev/null
+//
+func Stderr() *cobra.Command {
+	command := cobra.Command{
+		Use: "stderr",
+		Run: func(cmd *cobra.Command, args []string) {
+			if len(args) == 0 {
+				args = []string{cmd.Name(), "content"}
+			}
+			cmd.PrintErrln(strings.Join(args, " "))
+		},
+	}
+
+	return &command
+}

internal/cmd/root.go

@@ -22,6 +22,7 @@ func New() *cobra.Command {
 	/* configure instance */
 	command.AddCommand(
 		demo.Panic(),
+		demo.Stderr(),
 		demo.Stdout(),
 	)