[Modules] Colored output

[Xavier D. <xav...@gm...>]

The ability to graphically enhance some part of the produced output
has been added to improve readability. Among others, error, warning
and info message prefixes can be colored as well as modulepath, module
alias and symbolic version.

Color mode can be set to `never`, `auto` or `always`. When color mode
is set to `auto`, output is colored only if the standard error output
channel is attached to a terminal.

Default color mode could be controlled at configure time with the
`--enable-color` and the `--disable-color` option, which respectively
correspond to the `auto` and `never` color mode. This default mode
could be superseded with the `MODULES_COLOR` environment variable and
the `--color` command-line switch.

Color to apply to each element can be controlled with the
`MODULES_COLORS` environment variable or the
`--with-dark-background-colors` and `--with-light-background-colors`
configure options. These variable and options take as value a
colon-separated list in the same fashion `LS_COLORS` does. In this
list, output item that should be highlighted is designated by a key
which is associated to a Select Graphic Rendition (SGR) code [1].

The `MODULES_TERM_BACKGROUND` environment variable and the
`--with-terminal-background` configure option help Modules to
determine if the color set for dark background or the color set for
light background should be used to color output in case no specific
color set is defined with the `MODULES_COLORS` variable.

Output items able to be colorized and their relative key are:
highlighted element (`hi`), debug information (`db`), tag separator
(`se`); Error (`er`), warning (`wa`), module error (`me`) and info
(`in`) message prefixes; Modulepath (`mp`), directory (`di`), module
alias (`al`), module symbolic version (`sy`), module `default` version
(`de`) and modulefile command (`cm`).

For instance the default color set for a terminal with dark background
is defined to:

  hi=1:db=2:se=2:er=91:wa=93:me=95:in=94:mp=1;94:di=94:al=96:sy=95:de=4:cm=92

When colored output is enabled and a specific graphical rendition is
defined for module *default* version, the `default` symbol is omitted
and instead the defined graphical rendition is applied to the relative
modulefile. When colored output is enabled and a specific graphical
rendition is defined for module alias, the `@` symbol is omitted.

This change is available in git repository and will be included into
the next feature release v4.3. I suggest you to also look at this post
on the news timeline [2], which provides some colored output examples.

Regards,
Xavier

[1] https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_(Select_Graphic_Rendition)_parameters
[2] https://timeline.noticeable.io/cbzeG7wTvAIqj21zbUmx/posts/colored-output