idea: smart handling of new lints

[ericseppanen]

If I use a new clippy lint in Cranky.toml, but someone else hasn't moved to that Rust version yet, they will get an "unknown lint" error.

If I don't want to bump the crate's minimum supported Rust version, I should be able to add that lint anyway-- and cargo cranky should be able to observe what stable release I'm on and disable lints that are too new.

Would this require hardcoding a list of clippy lints and the version when they landed? That sounds a bit hard to keep up with.