def col_style(index, style, options={})
      offset = options.delete(:row_offset) || 0
      cells = @rows[(offset..-1)].map { |row| row[index] }.flatten.compact
      cells.each { |cell| cell.style = style }
    end
Set the style for cells in a specific column
@param [Integer] index the index of the column
@param [Integer] style the cellXfs index
@param [Hash] options
@option [Integer] :row_offset only cells after this column will be updated.
@note You can also specify the style for specific columns in the call to add_row by using an array for the :styles option
@see Worksheet#add_row
@see README.md for an example