Rename plot_posterior and improve it

[aloctavodia]

Other suggested names so far: plot_krushke, plot_book, plot_summarystats, plot_stats, plot_dist

I think plot_dist was the "winner" during the last discussion...

make it nicer:

• error when bad keys are present in kwargs
• make it work with datatree and dataarray
• add var_names argument and filter_vars