Add a warning for unrecognized keys

[jyn514]

In https://github.com/facebookincubator/cargo-guppy/issues/655#issuecomment-1147804389, I tried configuring haraki with transitive-excludes instead of traversal-excludes, which gave info: no changes detected with little explanation of what went wrong. It would be great if haraki could give a warning when it sees a key in .config/haraki.toml that it doesn't recognize.