Use postcss-load-config 3.0

[ai]

We updated cosmiconfig and the only breaking change is Node.js >= 10 (which is not a breaking change for this project, since PostCSS 8.0 was already limited Node.js to >= 10).

You can release postcss-load-config as a patch release.

[RyanZim]

Merged in #351; will release soon. Technically, this will be a minor release, since adds support for .cjs configs.

[RyanZim]

This was released in v8.1.0