windows: enable virtual terminal processing, fixes #169

[martinlindhe]

This PR supersedes #173 because I accidentally deleted local fork.

It is the same PR except x/sys bumped to 0.5.0.

[fatih]

Thank you @martinlindhe

[pellared]

Can we use NO_COLOR env var and NoColor = true global variable to opt-out?
In such configuration this code should not be executed.

#173 (comment)

[fatih]

the NO_COLOR env already changes the NoColor value. So we can just check for the NoColor variable and it'll cover everything.

[pellared]

So I guess it is just a matter of adding

if NoColor {
  return
}

right? 😉

@fatih let me know if you want me to address my own comments

[fatih]

The problem though is, it wouldn't work as expected. Imagine this situation:

1. You import color and enable NO_COLOR.
2. The init() is called, it check's NoColor, if it's false it will not set the configuration
3. After a while though, you set NoColor = false.
4. It still doesn't work, because init() will not run anymore. So we need a separate enviornment variable. NoColor is working fine as it's now, but it wouldn't work for opt-out.

[martinlindhe]

Regarding #2. init() is run after all var () blocks, so NoColor would be set before init() is called.

[pellared]

Indent Error Flow

Suggested change

	if err := windows.GetConsoleMode(out, &outMode); err == nil {
	outMode |= windows.ENABLE_PROCESSED_OUTPUT | windows.ENABLE_VIRTUAL_TERMINAL_PROCESSING
	_ = windows.SetConsoleMode(out, outMode)
	}
	if err := windows.GetConsoleMode(out, &outMode); err != nil {
	return
	}
	outMode |= windows.ENABLE_PROCESSED_OUTPUT | windows.ENABLE_VIRTUAL_TERMINAL_PROCESSING
	_ = windows.SetConsoleMode(out, outMode)

reference: https://github.com/golang/go/wiki/CodeReviewComments#indent-error-flow

[martinlindhe]

@pellared I sent yet another PR for your last 2 comments. See #187