Add Version module and --version

Description / Motivation

See #15 (closed)

Debate: is it worth the added steps during release (which we may easily forget)?

How to Test the MR Manually

dune exec test/unix/main.exe -- --version

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• Add tests in test/, if relevant.