Pretty print diagnostic summaries

summarise_draws() computes the summaries and outputs a tibble which is useful, but for notebooks it would be nice to have pretty printed output. I like the rstan::monitor() output

> monitor(samp, probs = c(0.25, 0.5, 0.75), digits_summary = 2)
Inference for the input samples (1 chains: each with iter = 2001; warmup = 1000):

     Q5 Q50 Q95 Mean  SD  Rhat Bulk_ESS Tail_ESS
V1 -0.9 0.2 1.5  0.2 0.8  1.14        5       15
V2 -0.9 0.2 1.5  0.2 0.8  1.15        5       18

For each parameter, Bulk_ESS and Tail_ESS are crude measures of 
effective sample size for bulk and tail quantities respectively (an ESS > 100 
per chain is considered good), and Rhat is the potential scale reduction 
factor on rank normalized split chains (at convergence, Rhat <= 1.05).

Specifically, for the possibility to define the number of digits and always rounding the ESSs.

It seems pillar is not flexible enough for formatting, so it seems monitor() style text output would be easier. It would be cool to have another function for pretty tables, but there are benefits for the simpler monitor() style output, too.

This is related to several earlier issues #44, #97, #120, #185 which mention only some parts of this issue

Start with summarise_draws() and review the related issues #44, #97, #120, and #185, along with the limitations of pillar mentioned here. Compare the requested digit control and ESS rounding with the rstan::monitor() style; done requires an agreed approach for the pretty-printed output and its interface.