Minesweeper

Add the mine counts to a completed Minesweeper board.

Minesweeper is a popular game where the user has to find the mines using
numeric hints that indicate how many mines are directly adjacent
(horizontally, vertically, diagonally) to a square.

In this exercise you have to create some code that counts the number of
mines adjacent to a given empty square and replaces that square with the
count.

The board is a rectangle composed of blank space (' ') characters. A mine
is represented by an asterisk ('*') character.

If a given space has no adjacent mines at all, leave that square blank.

Examples

For example you may receive a 5 x 4 board like this (empty spaces are
represented here with the '·' character for display on screen):

·*·*·
··*··
··*··
·····

And your code will transform it into this:

1*3*1
13*31
·2*2·
·111·