Expose colorable Stderr to use as writer

[achilleas-k]

Since printing colors on Windows requires defining the colorised writer (fmt.Fprint(color.Output, ...)), this PR exposes the equivalent colorised Stderr writer. So the following would be supported:

var green = color.New(color.FgGreen).SprintFunc()
var red = color.New(color.FgRed).SprintFunc()

fmt.Fprintln(color.Output, green("This is normal green text")) // Prints to os.Stdout
fmt.Fprintln(color.Output, red("This is normal red text")) // Prints to os.Stdout
fmt.Fprintln(color.Error, red("This is error red text"))  // Prints to os.Stderr

[fatih]

Thanks @achilleas-k