Support for NO_COLOR

[cmarqu]

There is a "competing" initiative to the CLICOLOR* variables implemented in #279:
https://no-color.org/, suggesting the variable NO_COLOR. Maybe modules should honor it too.

[xdelaruelle]

It reminds me of how standards proliferate. Joke aside, there is no problem to also honor this NO_COLOR variable.

Would you try a pull request for that? Work should be quite straightforward, based on code done for CLICOLOR (+manpage update, + few tests cases in testsuite/modules.00-init/140-color.exp)