libsmartcols: add scols_sort_table()

libsmartcols: add scols_sort_table()

 * add pointer to column cmp() function [scols_column_set_cmpfunc()]

 * allow to store per-cell application private data
   (to make it possible to sort tables on data independent on cell
    output data) [scols_cell_set_userdata() ...]

 * make it possible to access line cell by column
   [scols_line_get_column_cell()]

Sort and cmp() stuff based on patches from Shakur Shams Mullick.

Co-Author: Shakur Shams Mullick <shakursmullick@gmail.com>
Signed-off-by: Shakur Shams Mullick <shakursmullick@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>