Add rustfmt.toml option to disable formatting entirely

[casey]

Many developers have their editor set up to automatically run rustfmt. However, some projects don't use rustfmt, so when such a developer works on such a project, they'll produce a bunch of unwanted changes.

To avoid this, I suggest adding an option to rustfmt.toml that disables formatting entirely. For example a format option that defaults to true, but when set to false, disables formatting.

Then, a project that doesn't use rustfmt could just add a rustfmt.toml with:

format = false

And there would be no additional burden on contributors from having to configure their editor, or on maintainers from responding to PRs with unwanted format diffs.

[casey]

I just realized that there is such an option 😅

https://rust-lang.github.io/rustfmt/?version=v1.4.38&search=#disable_all_formatting