Make rustfmt config path an `Option`, remove argument if not provided

If no path to a rustfmt configuration file was provided, this program would default to rustfmt.toml, and force rustfmt to try and find it recursively in all folders, even if it did not end up finding it.

This patch makes the rustfmt configuration file path an Option, meaning that no default value is provided. If no file path is given, the --config-path option is never provided to rustfmt during its invocation.

(I also moved three lines of code that should be triggered only if rustfmt runs, they were not hidden by the suppressor flag in !6 (merged) )