function to plot coefficient/parameter estimates #4

It would be nice to have an easy way to plot parameters from an estimated model, analogous to a coefplot.

It would ideally support a scenario which would take a list of models & plot coefficients side-by-side, to facilitate model comparison task.

(This feature is inspired by the plots in Tim's ovarian cancer analysis)

The function would take a few inputs :

1. named dictionary of stanfit objects
2. parameter(s) to extract (should this be a regex?)
3. (optional) vector of names to map to parameters

The function will produce a plot showing coefficient values for each model given.