fix: add nil check for os.Stdout to prevent panic on Windows services

[majiayu000]

Fixes #250

Changes

• Add nil check in stdoutIsTerminal() before calling os.Stdout.Fd()
• Add initOutput() to safely initialize Output with nil check for os.Stdout
• Add initError() to safely initialize Error with nil check for os.Stderr
• Add nil check in color_windows.go init() before accessing os.Stdout.Fd()

[fatih]

@majiayu000 thank you for your fix. I pushed an additional test and renamed some of the functions. Going to merge it as it.