Option to skip crates whose Cargo.toml version is already published on the registry

[obi1kenobi]

@obi1kenobi I think if you could add an option to semver-checks that allows skipping the check of crates whose Cargo.toml version is identical to the latest published version, then this problem would be trivially solvable. You can't publish the same version on crates.io twice, so this would make it safe to skip the check in that case, and the first time it would fail is in the PR that changes the version number. Which is what we need.

Originally posted by @Finomnis in tokio-rs/tokio#5437 (comment)

Eventually, we want to implement a proper PR-centric workflow. In the meantime, a feature like this sounds like it could make using the current tool in PRs much easier.