[FR] add "index" column

[ildar170975]

Consider this config:

- name: some index
  data: _index
  start_from: 1

which creates:

Here:
_index - a directive to use an incrementing index (starting from start_from)
start_from - first value of the index (default 1)

This column cannot be selected for sorting. Also, sorting by a header is disabled for this column.
Values of the index are always incrementing.

A column with _index may be placed as the 1st, 2nd, ... whatever.
Also, imho there is no need to limit a number of these columns in a config (although a user may need only one).