when an empty cell is opened, all surrounding 0s should also be revealed

from Wikipedia: "If the player happens to click a mine square on their very first click, in most (not all) versions of Minesweeper the mine at this square is removed and a new mine is placed in the upper left corner. If there is already a mine in the upper left corner (or it was the square that the player clicked), a new mine is placed in the first (starting in the upper left corner then proceeding left->right, top->bottom) available empty spot of the board."

alternatively, generate board after user clicks the first square.

fix to allow lowercase or uppercase inputs, etc.