Minesweeper: Prevent first-turn fail, and other improvements

I probably should have made these changes into seperate pull requests, but w/e.

This closes !21 (closed)

Improvements include:
    - Make sure that the player cannot hit a mine on the first click.
    - Changing flagging behaviour:
        - The f and F keys are now combined into one; f now toggles flag status.
          (if current cell is flagged, then it is unflagged, if it is not, then flag)
        - Fixed bug where one could flag cells that had already been revealed.
    - Making flag and mine characters more descriptive
      (flag character is now 'F' and mine character is '*')
    - Making the background of the mine that lost you the game red.