Adding Color::AnsiColor is a breaking change

[fb39ca4]

Adding a variant to an enum that is not marked #[non_exhaustive] is a breaking change so 3.1.0 should have been released as 4.0.0.

e3bb575

Example of a dependent project which fails to build: tuna-f1sh/cyme#92

[spenserblack]

If you make a PR with the 4.0.0 version bump, I can merge that and trigger a new release with the appropriate version number. Additionally, that would be a good time to add the #[non_exhaustive] attribute so that we're safe to add any more variants in the future.

Unfortunately, I don't have the ability to yank the release with the invalid version number. @hwittenborn @kurtlawrence Are either of you able to do that? Or we could even revert e3bb575 if 256-color support isn't worth a breaking change.