Improve `grouped_tt` printing to show groups

[markfairbanks]

Closes #668

devtools::load_all(".")

df <- tidytable(x = c("a", "a", "b"), y = 1:3)

df %>%
  group_by(x, y)
#> # A tidytable: 3 × 2
#> # Groups:      x, y
#>   x         y
#>   <chr> <int>
#> 1 a         1
#> 2 a         2
#> 3 b         3