clean up extraneous tibble output

The current summary output looks like this:

> fit_hmc$summary(c("mu", "tau"))

# A tibble: 2 x 10
  variable    mean  median     sd    mad     q5     q95  rhat ess_bulk ess_tail
  <chr>      <dbl>   <dbl>  <dbl>  <dbl>  <dbl>   <dbl> <dbl>    <dbl>    <dbl>
1 mu       -0.0825 -0.0771 0.0140 0.0158 -0.101 -0.0665  1.97     2.03     18.1
2 tau       8.22    8.15   0.269  0.325   7.88   8.64    2.12     1.87     12.4

It also prints negative numbers in red. I can remove the red negative numbers with a global option:

> options(pillar.neg = FALSE)

I haven't been able to see how to get rid of the other output I consider to be extraneous:

1. the line saying it's a tibble (# A tibble: 2 x 10)
2. the line providing types: (<chr> <dbl> ...)
3. row numbers (1 for mu, 2 for tau)

P.S. This is low priority, especially if it's a huge pain to provide alternative tibble output.

The entry point mentioned is fit_hmc$summary(c("mu", "tau")); trace that summary method to see where the tibble is produced and how printing is delegated. Reproduce the shown output and define tests for omitting the tibble header, type row, row numbers, and optionally negative-number coloring; done means the requested summary display is covered.