Figure out a way to configure loaders from meta config

[d-fischer]

This is a follow-up to #288.

Currently, as I couldn't figure out a way to handle the loaders key in a non-js config file, I disabled it altogether for that use case.

This is an appeal to the community to supply ideas on how this could work.

A basic idea could be using the extensions of the default loaders, and loading separate files for custom ones. But I'm not sure how ergonomic that would be.