CSS Modules!

Major improvements

• @jakubrohleder added support for CSS Modules, a very long-standing request. Thanks Jakub! CSS Module support is enabled for all files with .module.(css|less|scss|sass) extension. #295

Minor (but awesome) improvements

• Some css module types (Less/Sass) weren't being minimized 0a667e0
• @adjohnson916 noticed we were using a private API of React Router #343
• Added check for processes listening to the port requested by gatsby serve-build similar to what we do for gatsby listen #345
• Set the NODE_ENV to production for node code (we were already doing this for Webpack modules but some Webpack loaders don't behave correctly without this fix #348

Breaking changes

None!