Support NO_COLOR

[rolandtritsch]

If you are using mill (with munit) in VSCode the build (and testing) output will (try to and fail to) display ANSI color codes in the Debug Console. munit supports the NO_COLOR env var. Mill obviously has the --color=false flag, but that means that I have to configure two things (instead of one).

If mill would support the NO_COLOR env var then it would all "just work".

The order of processing should be NO_COLOR first and the --color flag second (means with --color you can override NO_COLOR).

Happy to contribute a PR.

Thoughts?