Allow `--edition 2021` to be passed to rustfmt
Allow `--edition 2021` to be passed to rustfmt

This was added to Configurations.md in #4618, but the option wasn't
actually made available. This should let people who are using Rust 2021
on nightly rustc run `cargo fmt` again.

SingingTree/rustfmt,graydon/rust,graydon/rust,aidancully/rust,rust-lang/rustfmt,aidancully/rust,SingingTree/rustfmt,aidancully/rust,rust-lang/rustfmt,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust