Add flag to mark package.json as private

[andrewhamon]

trafficstars

This PR adds a --private flag to the build command, which is used to add a private field to the generated package.json.

This helps ensure that a package isn't accidentally published to npm.

Make sure these boxes are checked! 📦✅

• [x] You have the latest version of rustfmt installed

$ rustup component add rustfmt

• [x] You ran cargo fmt on the code base before submitting
• [ ] You reference which issue is being closed in the PR text

✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨