Use o.Writer if not nil to write output (#330)

Use o.Writer if not nil to write output

internal/driver/driver.go

 
 	// Output to specified file.
 	o.UI.PrintErr("Generating report in ", output)
-	out, err := os.Create(output)
+	out, err := o.Writer.Open(output)
 	if err != nil {
 		return err
 	}