Add skip-gitignore cli arg

[daggy1234]

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed

$ rustup component add rustfmt

• 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! 😄 ✨✨

This fixes #728 with a simple cli flag --skip-gitignore

[kyledecot]

Just started using wasm-pack and was wanting this feature–hooray for OSS 🎉

[daggy1234]

Thanks! Just waiting for the maintainers.

[AtHeartEngineer]

@daggy1234 @ashleygwilliams Whats the status of this?

[vankessel]

Looking for this feature as well

[stephanemagnenat]

I also need this feature!