Implement multi-category outcome models

[leeper]

prediction now supports multi-category outcome models (of various kinds) and offers the new category argument to specify which level of an outcome a la Stata:

margins xvar, predict(outcome(3))

prediction(model, category = 3)

So it should now be possible to cascade the category argument up to some new margins() methods and implement Stata-like interpretations.

[dnbarron]

Would it be possible to extend this to support clm as well as polr? Perhaps using something similar to the clm.to.polr function in the effects package? Happy to have a go at this myself if you're open to it.

This is a really useful package, thanks for providing it!

[leeper]

@dnbarron Yes, definitely. Basically once a model class is supported in prediction (see TODO list), it is then possible to implement it in margins, so it's just a matter of time. I'm open to PRs to add functionality but I do not intend to use the effects package as a dependency here.

[leeper]

Consolidating this issue to: #101