Add `to` option to `rollup-plugin-postcss`

[mgburns]

I'm trying to use the postcss-url plugin to copy fonts that are imported by my CSS into the build directory. In order for it to work properly it requires the to option for rollup-plugin-postcss to be set.

I was able to get it working locally by setting to: absMain.replace(EXTENSION, '.css') here: https://github.com/developit/microbundle/blob/22187fba8a2d404a9f3dc5db357e243cd45e8479/src/index.js#L495-L498

This is the postcss.config.js I'm trying to use:

module.exports = {
  plugins: {
    'postcss-url': {
      url: 'copy',
      basePath: '.',
      assetsPath: '',
      useHash: false,
    },
  },
};

Happy to provide more details, and thanks for your efforts on this great project!