Allow to group dependencies across languages

[tvdeyen]

I admit this is an edge case, but we have a gem (alchemy_cms) that has the JS code packaged in a npm package, while the ruby code is a gem. Both packages are equally versioned and live in the same mono-repo. Depfu already groups gems that belong together (Rails, Solidus) and live in a mono repo.

It would be nice if the npm package and ruby gem would be updated together like other grouped packages, even if the language is not the same.

Tricky one, but cool nevertheless ;)