PNG Optimization

I want to add seamless optimization to the sprite creation process, but without blowing everything up due an external dependency missing. I don't know nothing about PNG compression and optimization, but I guess that [image_optim](https://github.com/toy/image_optim) is a great facade for all this universe.

## TODO

* [x] `image_optim` dependency.
* [x] Safeguards against missing optimizers.
* [x] Emit a warning when optimization can't be done.