Move Pixi.js dependency to node modules

[abaybektursun]

Currently, Metacar requires you to have script tag in html to import Pixi.js dependency

<script src="https://cdnjs.cloudflare.com/ajax/libs/pixi.js/4.7.1/pixi.min.js"></script>

I tried removing it and npm installing pixi.js package, but Metacar did not work:

metacar.min.js:1 Uncaught TypeError: Cannot read property 'Application' of undefined

Need to be able to install Pixi.js from npm.

[abaybektursun]

This probably should be merged with #29