Support running tests in webpack

[andywer]

Provide this as an opt-in feature after implementing #32:

Build testing code using webpack, so you can use css-loader, sass-loader, vue-loader, ... in your test code and your components.

Usage

npm install shutter shutter-webpack
npx shutter test --webpack

How to tell shutter about the webpack configuration: TBD.

Implementation (Research necessary)

Running webpack with target: "node" and extracting the stylesheets into a separate CSS file that can be submitted to shutter.sh should do the trick.

[dsseng]

Great idea! It will be very useful for many projects.