Add `--check` command-line flag to `cargo-fmt`

[topecongiro]

cc @calebcartwright

[calebcartwright]

👍 - will open a PR for this later today. BTW should we target PRs to merge into master now that 1.4.10 has been release or still target the rustfmt-2.0 branch?

[topecongiro]

Thanks!

BTW should we target PRs to merge into master now that 1.4.10 has been release or still target the rustfmt-2.0 branch?

We can target PRs against the master branch. I will mention this in the tracking issue.