Update defaults to match create-react-app

[arunoda]

Create React App is setting some default build infrastructure for React apps. We had an issue of deciding what's the default config is.

Now we support, create-react-app's settings as the defaults. That means, we could use react-storybook with create-react-app based projects without doing any webpack configurations.

This comes with following core changes:

• Add postcss based CSS loader.
• Add file-loader for images and common types.
• Add url-loader for shorter media files.
• Do not pre-build manager(storybook UI) bundle.
• Continue support for babel's stage-0 preset and add es2016 preset.

[willianjusten]

Great Work @arunoda, this will help me a lot! \o/