Feature Request: Add a subcommand to check changes

[akshettrj]

It would be nice if a subcommand could be added to the godmod2nix binary which basically checks whether the content of gomod2nix.toml needs to be updated beacuse of changes in go.mod and go.sum.

This would come handy in CI and for things like git pre-commit hooks.

[akshettrj]

I am willing to make a PR as well if the maintainers think this will be acceptable