Also, it's now more compliant with the no-color.org spec:
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.
so one can now temporarily override $NO_COLOR=1 in the environment by
setting it empty: NO_COLOR= hledger ...
