Enable CSS sourcemaps in development

Created by: JaKXz

Hi! Firstly, of course, thank you for this amazing project! 👏

I've seen the comments on sourcemaps: #109, #139 (closed), #494 (closed), #343 (closed), etc. but didn't see one about enabling CSS sourcemaps. I thought I'd make a PR first but I wanted to discuss first and get to grips with this repo structure.

Since there aren't breakpoints to worry about, this could be a very simple enhancement, e.g:

loader: 'style-loader!css-loader?'.concat(isProduction ? 'minimize' : 'sourceMap')

where const isProduction = process.env.NODE_ENV === 'production'. Apologies if I am duplicating / creating noise, I thought I'd searched thoroughly.