Readme tests

[Dushistov]

skeptic add unnecessary build time dependencies, so I add only CI test which check that code in README.md is valid.

[punkstarman]

I really like this idea.

Why not add this to the build proper?

This roundabout way of opting into this build step will break very easily upon changes to Cargo.toml in ways that don't seem obvious to me. It would be less of a problem if the ci/skeptic.sh script were more robust, maybe even using a TOML-savvy library instead of a combination of sed and appending.

I don't see much of a downside to having this step on every build. I see the upside that the contributor won't wait for CI to finish before noticing regressions in the documentation examples.

[RReverser]

Looking at the upstream issue about build dependency budziq/rust-skeptic#60, looks like someone created an alternative that can be added just as a dev-dependency: https://docs.rs/docmatic

This seems much nicer as it doesn't impose an extra dependency on downstream users of the crate, while still enables testing of README on both CI and contributors' machines.

[RReverser]

I've created an alternative PR with fixes to README cherry-picked from here (thanks @Dushistov) but using docmatic for running tests: #90