copy/move rhat() and neff_ratio() to rstan(arm)? #119

I was looking for functions to extract rhat and the ratio for number of effective samples, and found those in the brms-package for brmsfit-objects, but not in rstan or rstanarm for stanreg-objects (where I would expect those functions to be).

Instead, I later realized that these are in the bayesplot-pkg. Wouldn't it make more sense to have the functions (also) in rstan/rstanarm as well?