Support postcss.options.plugins being a function

[barraponto]

We currently support postcss options being a function, but not options.plugins.
This should be mostly harmless -- I wanted to provide a test but we don't currently test support for any postcss plugin, do we?

[barraponto]

btw, i'm trying to follow the same pattern as postcss-loader: https://github.com/postcss/postcss-loader#plugins

[gucong3000]

const loadOptions = require('postcss-load-options/lib/options.js');
const loadPlugins = require('postcss-load-plugins/lib/plugins.js');

plugins = loadPlugins(inlineConfig);
options = loadOptions(inlineConfig),

i think it is better to keep callback API sync with postcss-load-config

[barraponto]

Well, it's not really my use case, but I can relate to a plugin configuration like this:

{
  loader: 'postcss-loader',
  options: {
    plugins: (loader) => [
      require('postcss-import')({ root: loader.resourcePath }),
      require('postcss-cssnext')(),
      require('autoprefixer')(),
      require('cssnano')()
    ]
  }
}

Copied from https://github.com/postcss/postcss-loader#plugins