Sort order in summarytable and listingtable when groups are missing

[jkrumbiegel]

Fixes #22

This sorts the row groups and column groups separately, so sort order in each of those isn't affected by whether all combinations exist.

julia> df = DataFrame(value = ones(6), A = ['c', 'c', 'c', 'b', 'b', 'a'], B = [4, 2, 8, 2, 4, 4])
6×3 DataFrame
 Row │ value    A     B     
     │ Float64  Char  Int64 
─────┼──────────────────────
   1 │     1.0  c         4
   2 │     1.0  c         2
   3 │     1.0  c         8
   4 │     1.0  b         2
   5 │     1.0  b         4
   6 │     1.0  a         4

julia> summarytable(df, :value, rows = :A, cols = :B, summary = [sum])

Before:

After: