Consideration for mkdocs get-deps regarding the [ansi] extra

[oprypin]

Hi!
I am thinking how to make this plugin work out of the box with mkdocs get-deps.
Most people would probably want it to suggest markdown-exec[ansi] but it currently gives you just markdown-exec and there's no way to make it include [ansi].

Here are some things that could be done:

1. Just make pygments-ansi-color a default dependency of this plugin - any big reason why not?

2. Add a new config that people should use :

    - markdown-exec:
        ansi: required   # one of: [off, required, auto], default: auto

   (bonus compared to the current behavior: it also guards against the ansi feature silently not working - the required setting would break on missing import)

   in which case we can put this option into extra_dependencies in https://github.com/mkdocs/catalog

3. Somehow just directly put markdown-exec[ansi] into https://github.com/mkdocs/catalog

Separately, I also noticed by reading the implementation that the CSS file is currently also always included into the final site, whether or not the ansi feature was successfully enabled.

[pawamoy]

No big reason why not including the extra dep by default, other than reducing deps. Option 3 would be a short term solution, but I can implement option 2 instead asap 👍

[pawamoy]

Done. We can now update the catalog. Will send a PR.