Add pixi.js facade

[fabienbk]

JS framework name: Pixi.js v4 Super fast HTML 5 2D rendering engine that uses webGL with canvas fallback
Facade repo: https://github.com/fabienbk/pixi-scala-js
SBT dependency: "com.fbksoft" %%% "pixi-scala-js" % "1.0"
Facade demo app: http://asteroids.fbksoft.com/

Thanks a lot!