Add menu, difficulty selection and endless mode

[Jujuyeh]

Now you can select different game modes (with Left, Right arrows and Enter):

• Easy mode: one tile adds one second
• Medium mode: three tiles adds one second
• Hard mode: five tiles adds one second
• Endless mode: there's no timeout. Instead, it shows elapsed time. Add endless mode #1
• Exit (To exit the game)

Once you finish a game, it will show up "Press [Ctrl + C] to return to the menu" to start another game. #2