considering stderr in UsageString

command.go

@@ -384,13 +384,22 @@ func (c *Command) Help() error {
 	return nil
 }
 
-// UsageString return usage string.
+// UsageString returns usage string.
 func (c *Command) UsageString() string {
+	// Storing normal writers
 	tmpOutput := c.outWriter
+	tmpErr := c.errWriter
+
 	bb := new(bytes.Buffer)
 	c.outWriter = bb
+	c.errWriter = bb
+
 	c.Usage()
+
+	// Setting things back to normal
 	c.outWriter = tmpOutput
+	c.errWriter = tmpErr
+
 	return bb.String()
 }