Hey there, thanks for making this plugin!

I noticed an incompatibility with the postcss-simple-vars plugin. Specifically the option to set default variables

postcss.config.js

const colors = {
    blue: '#056ef0'
}

module.exports = {
  // ...
  plugins: {
    // ...
    'postcss-simple-vars': {
      variables: colors
    },
    // ...
  }
}

^ this all works fine in webpack and I can use $blue in my css files. However, attempting to bundle with rollup-plugin-postcss produces this error:

(postcss plugin) Error: Undefined variable
color: $blue
           ^^^^^^

And my rollup config:

import postcss from 'rollup-plugin-postcss';
plugins: [
  // ...
  postcss({
    extract: `dist/${name}.css`,
  })
]

Any ideas on why these default variables would not be supported in rollup-plugin-postcss, but works fine in webpack?