Colour blind mode

[ewels]

It would be great to have a style config set that is colour-blind friendly which can be invoked by the presence of an environment variable.

Then anyone who has that env var set in their terminal will have colour-blind friendly CLI output from every tool that uses rich-click (without any need for the developers of those tools to do anything).

Could work well with the planned style sets - maybe generalise to end users being able to set global configuration for any rich-click tools.

[ewels]

Oops, didn't see #212. Closing this as it's a duplicate.