Hot Module Replacement (HMR)

[nstepien]

Would it be possible to add support for HMR?

For example

postcss({
  hot: process.env.ROLLUP_WATCH === 'true', // new!
  extract: true,
  minimize: process.env.NODE_ENV === 'production'
})

rollup-plugin-hot-css (https://github.com/PepsRyuu/rollup-plugin-hot-css) does support it, but it's not as complete as rollup-plugin-postcss feature-wise. Maybe rollup-plugin-hot-css should be merged into rollup-plugin-postcss? @PepsRyuu WDYT?