Respect the NO_COLOR environment variable

[suve]

The NO_COLOR environment variable is an informal standard which states:

All command-line software which outputs text with ANSI color added should check for the presence of a NO_COLOR environment variable that, when present (regardless of its value), prevents the addition of ANSI color.

How it should work:

1. If the --colour command-line option is explicitly specified, its value is used.
2. If the --colour option is not specified (or specified as AUTO), then:
   a. Check if the NO_COLOR envvar is present. If it is, disable coloured output (NO).
   b. Check if stdout is attached to a terminal (isatty()). If it is, enable coloured output (YES).
   c. Otherwise, disable coloured output (NO).

The code, as it stands currently, performs steps 1, 2b and 2c, so this is a matter of adding the extra logic described in step 2a.