Add --path and --gemfile opts to package

This PR is meant to address this feature request: https://github.com/bundler/bundler-features/issues/23

Seemed like all I needed to do was add those options. The behavior for those options seem to be covered by the logic in `install`. Let me know though if there's anything else I need to do.

Should I add specs for these? If so, where do I put it? Couldn't figure out where to put them.