Mixed effects cox models

[dchiu911]

I couldn't find any tidy models implementation of coxme::coxme() in either broom or broom.mixed. Valid to add?

[topepo]

Maybe. We are just now incorporating both mixed models and survival methods. For the time being, I want to keep it fairly simple.

It would be helpful if you contributed a small vignette here (like the Cox model one) that described it, any oddities, and the package syntax for modeling/prediction.