feat: auto-detect CI from environment

[owenvoke]

What:

• [ ] Bug Fix
• [x] New Feature

Description:

I was looking at this PR on the docs earlier, and thought that it would be quite nice to add auto-detection for CI environments.

The majority of CI services (GitHub, GitLab, Travis, Bitbucket, CircleCI, etc.) provide a CI environment variable that is set to true, and this would basically allow us to auto-detect based on that.

[!NOTE] My only thought is that this automatically disables the Only plugin, which someone may want to use in CI.

I'm not 100% sure about whether people do use it in CI much, but just thought I'd bring it up for discussion.

On the flip side, I do like being explicit, but having this might prevent excessive CI time usage when people forget to include --ci. 🤷🏻