This decision was part of rust-lang#10472

-  It looks like all commands except `cargo-add` are using the prelude to
  define `--features`, so this should hit them all.
- I've updated completions to the best of my knowledge.
- It appears we don't need to update man pages or other documentation.
- I'm assuming we don't add tests for every sort flag but rely on clap
  to have coverage to ensure it works.

The manual change is to `src/doc/man/includes/section-features.md`, the
rest is code-generated.