[CRIT] Show how explosions are progressing

[dplassgit]

I've played the game enough times that I can beat CRIT on the hardest level, but I still don't quite understand how the chain reactions work, especially when they are complicated and appear to be jumping all over the place. On the Tandy 200, it would be possible to change the grid lines around an exploding cell to show arrows pointing to the surrounding squares so that one can get a sense of where the chain reaction is going to go. (Once the surrounding squares are updated (which may be a while if it is a long chain reaction), the gridlines would be restored.)