Feature: multiple passes / process generated assets

[miltoncandelero]

The ability to run plugins on generated assets rather than the input ones.

Motivation:

• Compressing images exported by texture packing
• Deduplicating manifest: Checking all created files and if two have the same checksum, keep only one copy and reference multiple times in the assets plugin

Current workarround:
Having multiple configuration files and multiple folders and run assetpack sequentially leapfrogging the assets until they reach the final state