Allow arbitrary grid sizes

[wlof]

At the moment, the grid size is fixed by the terminal size.

Ideally, Conway's Game of Life should be played on a large (potentially infinite) grid.

This feature would require dissociating the grid size from the view size, which would be a good thing anyway.

[wlof]

Grid size and view size now exist independently as of 0.2.0.

Grid size is fixed at 100x100 for now, but we only need to add some command-line arguments to let the user specify its own grid size.

[wlof]

Done as of 0.3.1.