Build requires PostCSS @ 8 support #670

<!-- Please don't delete this template or we'll close your issue -->

<!-- Before creating an issue please make sure you are using the latest version of Saber. -->

<!-- Issues which contain questions or support requests will be closed. -->

<!-- Please ask questions via following several ways. -->

<!-- https://chat.saber.land/ -->

<!-- https://stackoverflow.com/questions/ask?tags=saberjs -->

Bug report

The dev server (saber) fails to compile with the message:

ERROR in ./node_modules/prismjs/themes/prism-tomorrow.css (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-3-1!./node_modules/@egoist/postcss-loader/src??ref--5-oneOf-3-2!./node_modules/prismjs/themes/prism-tomorrow.css)
Module build failed (from ./node_modules/@egoist/postcss-loader/src/index.js):
Error: PostCSS plugin autoprefixer requires PostCSS 8.
Migration guide for end-users:
https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users
    at Processor.normalize (/Users/anc/code/cottage/node_modules/postcss/lib/processor.js:167:15)
    at new Processor (/Users/anc/code/cottage/node_modules/postcss/lib/processor.js:56:25)
    at postcss (/Users/anc/code/cottage/node_modules/postcss/lib/postcss.js:55:10)
    at /Users/anc/code/cottage/node_modules/@egoist/postcss-loader/src/index.js:145:12
 @ ./node_modules/prismjs/themes/prism-tomorrow.css 4:14-154 14:3-18:5 15:22-162
 @ ./saber-browser.js
 @ ./.saber/extend-browser-api.js
 @ ./node_modules/saber/vue-app/create-app.js
 @ ./node_modules/saber/vue-app/entry-client.js
 @ multi ./node_modules/saber/vue-app/entry-client.js

Steps to reproduce

1. Brand new saber project
2. npm run dev

<!-- If you are reporting a bug that can ONLY be reproduced on your repository, PLEASE provide this repo link. That takes guessing work out of the way and saves us time. -->

<!-- You can also use this CodeSandbox starter https://bit.ly/2IlcpNF -->

What is expected?

Saber would compile

What is actually happening?

Post CSS complains about not having the minimum required version

Other relevant information

• Result of running saber -v: 0.11.7
• Browser version (optional): N/A
• Is Saber a global or local install? Local
• Which package manager did you use for the install? npm
• Does this issue occur when all plugins are disabled? Yup