Should not suggest npm publish if the package is private

[watilde]

Hello! I'm using standard-version in my private package, and the private package never is published. So my request is if the package.json has private:true, then the result doesn't display the npm publish.

$ standard-version patch
✔ bumping version in package.json from 1.0.4 to 1.0.5
✔ outputting changes to CHANGELOG.md
✔ committing package.json and CHANGELOG.md
✔ tagging release 1.0.5
++ ℹ Run `git push --follow-tags origin master` to publish
-- ℹ Run `git push --follow-tags origin master; npm publish` to publish

Thanks.