When printing grouped data frames use thousands separator when printing number of groups

[rbdixon]

This should be consistent with the usage of thousands separators when printing the number of row numbers.

x = tbl_df(
  data.frame(n=1:1000)
) %>%
  group_by(n)

print(x)

Prints:

Source: local data frame [1,000 x 1]
Groups: n [1000]

       n
   (int)
1      1
2      2
3      3
4      4
5      5
6      6
7      7
8      8
9      9
10    10
..   ...

PR coming soon to add big_marks() to print.grouped_df().