Support 'g CTRL-A'

[garrett-hopper]

'g CTRL-A' is very useful for creating lines of increasing numbers.

                                                        CTRL-A
CTRL-A                  Add [count] to the number or alphabetic character at
                        or after the cursor.  {not in Vi}

                                                        v_CTRL-A
{Visual}CTRL-A          Add [count] to the number or alphabetic character in
                        the highlighted text.  {not in Vi}

                                                        v_g_CTRL-A
{Visual}g CTRL-A        Add [count] to the number or alphabetic character in
                        the highlighted text. If several lines are
                        highlighted, each one will be incremented by an
                        additional [count] (so effectively creating a
                        [count] incrementing sequence).  {not in Vi}
                        For Example, if you have this list of numbers:
                                1.
                                1.
                                1.
                                1.
                        Move to the second "1." and Visually select three
                        lines, pressing g CTRL-A results in:
                                1.
                                2.
                                3.
                                4.