TODO

* Set a max ball number conf value. This avoids the game to be over crowded.
OK

* Handle bonuses and maluses.

They appear on the board every "random number between 1 and 10" seconds.
OK

They need to be catched by the paddle.

They have a lifetime of maybe 10 seconds.
OK

    Extra ball (make an additionnal ball appear in the game area's center)
    Increase paddle speed
    Decrease paddle speed
    Increase opponent's paddle speed
    Decrease opponent's paddle speed
    Increase paddle size
    Decrease paddle size
    Increase opponent's paddle size
    Decrease opponent's paddle size
    Increase ball speed when the ball hits the paddle
    Bullet (if opponent is hit, increase the shooter's score)
    Invert controls (up becomes down and vice versa)

A large paddle can catch a speed bonus for example, but bonuses and maluses
can't be cumulative:
    If the paddle is already large it can't be larger
    If the paddle hasn't fired a bullet it can't grab anymore bullets

* Use canvas instead of html/css

* Use images to represent paddles, projectiles, bonuses.

* Use a nice background image.

* Integrate sounds
    When paddles hit balls
    When balls hit walls
    When bonuses are catched
    Background sounds

* Keys have to be configurable. By default:

        Left player     Right player
Up        i               up arrow
Down      k               down arrow
Shoot     l               left arrow