Support for `NO_COLOR` proposal

[seebi]

Expected Behavior

As described at https://no-color.org/, setting NO_COLOR to any value should disable color output at all.

Actual Behavior

Every application need its own solution for color disabling.

I think this behavior should be added to Click itself because its a reasonable default.

[jcrotts]

See this issue for discussion on a similar request.
This may have more of a rationale behind it, but in general I don't think click benefits from this sort of customization.

[davidism]

I find myself agreeing with ggreer/the_silver_searcher#1207 (comment), which is linked from the page you linked. It's not clear that this standard is a standard or that it's the correct thing to do.

As Jay points out, this has already been discussed in #1090 and we reached similar conclusions then. If an application wants to support controlling the color with an env var, it's straightforward to add.

if __name__ == "__main__":
    color = os.environ.get("CLICK_COLOR")
    color = color in {"1", "true"} if color is not None else None
    cli(color=color)