New visualization mode

[wlof]

Right now the view of the cells is pretty basic: all live cells are represented by the same character (*). Cells born in the current generation are drawn in green, others in white.

Here's a more fancy visualization mode:

• use different characters according to the "fate" of the cell: will it die of isolation, overcrowding, or will it survive?
• use different colors according to the age of the cell: e.g. different colors for cells born 0, 1, 2, 5+, 10+ generations ago

This requires changes in the game implementations to compute and keep track of these new informations (fates and ages).

[wlof]

Done in 0.4.0.