Add .rustfmt.toml to configure Rustfmt.

[mawallace]

An empty .rustfmt.toml file will configure Rustfmt with the default settings (which seems to match how the project is formatted).

Without this, Rustfmt will

1. Look for configuration in parent directories (which will likely exist if someone has vendored this dependency); or
2. Look for global configuration

If either exist, it makes it more difficult to contribute to this project, since IDEs will auto-format according to that config and create spurious changes.