Support Phaser 3 exporter

[sylvainpolletvillard]

This is an attempt to make assetpack usable on Phaser 3 projects. I don't think it covers all the specificites of the Phaser 3 JSON exports, but it has been enough to make it work with my game.

I don't have a lot of knowledge about Pixi vs Phaser JSON format, and found it quite hard to find quality documentation about it. But I hope someone would find it useful.

fixes #53

[Zyie]

hey, just got back from holiday, so i will review this soon!