Refactor/postcss

[trezy]

Description I've converted the project to just using PostCSS, eliminating the individual usage of Autoprefixer and CleanCSS in favor of using the same libraries as PostCSS plugins. Here's the build tool conversion chart:

Original script	PostCSS plugin replacement
build:sass	postcss-node-sass
build:autoprefix	postcss-preset-env
build:cleancss	postcss-clean

Caveats Currently, external source maps aren't being generated. I'm working on figuring out why that is.