Have a separate category in the report for crates using nightly features

[scottmcm]

In https://crater.rust-lang.org/ex/pr-82322, the vast majority of the crates that were broken were those using #![features]s. But since it's allowed to break such crates anyway, it would be nice to separate them into a different category, or mark them in some way, to make it easier to concentrate on the stable regressions that are the most important ones.