sass @import directives is ignored

Hi again.

We widely using '@import' directive in our project and it seems to be not supported for now in babel-plugin-react-css-modules. I also dig sources and found nothing in postcss pipeline that could process it.

**Would do you like to integrate postcss plugin for @import handling?**

A likely candidate is postcss-import plugin. It has one problem: it is asynchronous. So it break your flow, because of `lazyResult.root` usage. It is only works with syncronous plugins (I also unearth that this is not recommended approach: http://api.postcss.org/LazyResult.html#root).

Sync mode will not be implemented according to postcss/postcss-import#180 and postcss/postcss-import#293 but there is two workarounds:

1. Use postcss-import 7.x.x which is syncronous. I try and it works fine for a quick look.
2. Take a look at https://github.com/css-modules/postcss-icss-import which is reffered in postcss/postcss-import#293. But as author said it is in beta now and not yet working.

What do you think about?

PostCSSパイプラインとlazyResult.rootの使用箇所から始め、次にissueで言及されているpostcss-importとpostcss-icss-importのオプションを確認します。同期処理と非同期処理の制約を比較し、文書化されている回避策を検討します。完了条件は、プラグインの既存のフローを壊さずにSass @importディレクティブが処理されることです。

索引モデルが issue の本文から書いたものです。