Extend to ivreg objects

[dswatson]

Would it be possible to extend interplot to ivreg objects? I get an error when I try to run this due to the fact that arm::sim does not work with ivreg models. Extracting the second regression from the 2SLS and running the plot on that alone "works", but standard errors are misleading here. I can patch something together for my particular application but I think there might be more general interest in this functionality.