[feature] Assert `rust-version` field in `Cargo.toml` matches MSRV in README

[lopopolo]

As an example use case, the https://github.com/artichoke/strftime-ruby repository calls out the crate's MSRV in the README.md file and also sets the rust-version attribute in Cargo.toml.

I'd love to be able to assert these values are the same, similar to the version matcher.

[mgeisler]

Hey Ryan, thanks for the idea! I think that would be a great addition to the library.

Would you be up for implementing it? There is already code which parses the Cargo.toml file, so that could be used as a starting point.