Add to the UI

Stumbled across your project when I was looking for minesweeper I could play easily on linux. The game is real solid, nice work. I added some interface elements to set the game board size, including the classic windows preset sizes for beginner, intermediate, and expert. I also added a timer and keep track of best times using localStorage.

I think I've tested it all reasonably thoroughly, but this is literally my first pull request, so I'm a little new here. Anyway, great version of this game.

Bryan