Ship babel-plugin-react-docgen with storybook

[arunoda]

The repo: https://github.com/kadirahq/babel-plugin-react-docgen

With this, we could get more meta data information about React classes in runtime.
For an example, you could get more info any React class with:

import MyButton from '../components/button';
console.log(MyButton.__docgenInfo)

This will help us to build a huge set of storybook addon which improve the dev workflow.

Performance Cost

This might carry some performance cost since it parses classes with react-docgen.
But we only do this only for files with React classes. We also utilize babel caching dir.
So with all, this won't be adding much overhead.