check for NO_COLORS before enabling colors

[naturallymitchell]

use sys env vars according to https://no-color.org/ see jazzdotdev/jazz#162

[sineemore]

When standard output is a file handle (or a pipe) colors will be written directly to it, and now all your logs will be with escape sequences.

Colored output is only meaningful when outputting to terminal.
NO_COLOR is a way to tell program not to use colors in terminal.
But I guess log files should be without colors (escape sequences) anyway.

isatty(3) can be used to determine if a file descriptor refers to a terminal.

(btw, you can search GitHub for code with "isatty color" to reveal the problem.)