update to postcss 8

[cowills]

Hello, my team at work uses this plugin and we are hoping to update to postcss 8 in the next few months. The plugin migration looked straightforward so i took a shot at it following this migration guide and using this other plugin PR as an example:

https://evilmartians.com/chronicles/postcss-8-plugin-migration
https://github.com/Genuifx/postcss-pxtorpx-pro/compare/dev-8

• moved postcss to a peerDependency/devDependency
• updated the plugin module.exports syntax
• moved plugin code into the new Declaration and AtRule functions
• replaced decl.parent.insertAfter with decl.cloneAfter

resolves #66

[yangmingshan]

@cuth Could you spent a little time to review this PR?

[cuth]

Thanks for the ping. I'll review this soon.

[cuth]

Thanks!

[cuth]

Published 6.0.0 for postcss 8

[yangmingshan]

Seems you miss the change log, just a reminder 😀. Thanks for your time.

[cuth]

@yangmingshan you're right, I did. Definitely should call out breaking changes. Thanks!

@yangmingshan would you be interested in being a maintainer? I've gone through too many life changes and don't have much time for open source right now.

[yangmingshan]

Yes I'm interested. My npm username is also yangmingshan.