Consider adding Cargo.lock to version control

[TomaSajt]

As per the default comment in the .gitignore file, it is recommended that if you're building a binary and not just making library, that you remove Cargo.lock from .gitignore. https://github.com/magiclen/markdown2html-converter/blob/665545f1cd5240c14cf9bf468e40551b6cb146b7/.gitignore#L89-L91

Having Cargo.lock present will make the build process more reproducible as the used dependency versions won't be different between people.

Having access to Cargo.lock will also help me with maintaining this package inside Nixpkgs (my PR), because otherwise we have to vendor the file ourselves (which increases the size of our repository by a considerable amount, which we want to avoid if possible)