We no longer need the Ubuntu LTS restriction, so now the only only
constraint I'm aware of is Firefox's policy. Fortunately, that tracks
the latest stable delayed by only two weeks. So this puts is at
Rust 1.29 now.
* Add a comment to .rustfmt.toml explaning why it's here.
* Use `<details>` for specialized information in README.md.
* Describe a more elaborate issue-labelling system.