work out how to arrange loader rules in a sensible way

[notmodified]

make it so we dont have to pull everything out to change postcss lists

[notmodified]

i think this (or something very close) https://medium.com/making-internets/webpack-configuration-done-right-86c325a6927f might be the answer.

I'll have a fiddle.

[angry-dan]

I had a first go at this in #24. I don't think it's quite ready yet, but it's a start, and I added a snapshot test to prove that the config didn't change.