Consider checking Cargo.toml snippets for correct version

[clarfonthey]

For example, this crate itself has the block:

[build-dependencies]
skeptic = "0.9"

[dev-dependencies]
skeptic = "0.9"

Let's say that the crate has a breaking change and updates to 0.10, but you forget to change README.md to reflect this. It'd be nice if skeptic could somehow fail and indicate to the user that README.md should be updated to reflect the latest version.

I've had a few crates stumble upon this mistake (including several of my own) and it'd be nice if skeptic could keep track of this too!

[budziq]

This should be doable in a basic form as we started to use cargo_metadata to solve some more tricky linkage scenarios.

At the moment I don't think it would be possible to (easily) match against features or dependencies other than crates.io ones (path/vcs), but a basic form that would check if cargo.toml snippet is semver compat with build deps would be possible. Would it be enough @clarcharr ?