Add support to Animations for Reversing the play

[eonarheim]

Context

In the current state, you must reverse the animation frames to reverse the order of animation. See discussion

// Pseudo code
const forwards = ex.Animation({...}); // animation that ends
const backwards = forwards.clone();
backwards.frames.reverse(); // reverses in place, okay because clone() creates a new frames list so the 'forwards' animation should be fine

const actor = new ex.Actor({...});
actor.graphics.use(forwards);
forwards.events.on('end', () => {
   actor.graphics.use(backwards);
});

Proposal

Add a property or method to reverse the direction of play.

Possilbly? API design notes welcome

const animation = new ex.Animation({...});
animation.reverse(); // reverse the play direction

animation.direction; // Forwards or backwards sentinel