Implement the MasterMind game.

The computer "thinks" a string with 4 different digits. You guess which digits. For every digit that matched both in value, and in location the computer gives you a *. For every digit that matches in value, but not in space the computer gives you a +.

Try to guess the given number in as few guesses as possible.

Computer: 2153
You:      2467  *
You:      2715  *++