Slovenian translation added

Hello, I'm proposing adding the Slovenian translation (sl_SI) as well. SemVer v2.0.0 translated and reviewed/re-read few times by me. Local installation works fine as well. If we need another review-er I maybe get someone else to check it out but there should be everything working ok. I'm also planning to add patches when English origin gets updated. Thanks for considering adding it to semver.org. Kind regards, Peter.