Create a way to declaratively exclude modules/paths from babel-loader

[dlmr]

Created by @PAkerstrand

We have some vendor files that are already webpacked, babelified etc. These files have a tendency to break if they run through babel-loader a second time. For the time being, we solve it like this in an action in our roc.config.js-file:

Ref: rocjs/roc-package-webpack#10