Add support for NO_COLOR

[LukeSavefrogs]

From the NO_COLOR website:

Command-line software which adds ANSI color to its output by default should check for a NO_COLOR environment variable that, when present and not an empty string (regardless of its value), prevents the addition of ANSI color.

The program should check for the variable and eventually disable color output in the help file and other places.
Optionally, it could even have a --no-color flag and could check if the program is running in an interactive TTY; if so it could enable colors, otherwise disable them, similar to how grep works.