Ability to configure processOutput to be executed on afterEmit

[agelico]

Got the same problem as described here, but with v7.1.1. Had to downgrade to 7.0.0.

It's important for my project to have access to the emitted files in processOutput because some files are generated by other webpack plugins. Adding them in processOutput to assets.json is the best way, but it is impossible when the function is called on emit instead of afterEmit.

As there were previous issues with same request, maybe it's worth doing a separate setting for it?