[m-dev.] MERCURY_ENABLE_COLOR

[Zoltan Somogyi]

At the moment, you can use environment variables to specify
a color scheme or to disable the use of color, but not to enable
the use of color. I don't think that just doing "unset NO_COLOR"
is an adequate solution, because you may want to enable color
in Mercury diagnostics without necessarily enabling it for any other
program.

Would people object if I added code to insert --enable-color-diagnostics
into the command line if the envvar MERCURY_ENABLE_COLOR is set
and nonempty, at the position where we now insert its opposite
if NO_COLOR is set and nonempty?

The reason I ask is that I think the color diagnostics are now as ready
as I can make them, so I would like to switch them on by default. For that,
I need to document them, and for *that*, I would like the system to be
documented to be in its final form.

Zoltan.