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.