v3.0.0

There are a few breaking changes in this release and requires upgrading React to 16.3.

• The disableOverlay prop is deprecated and will be removed in the next major release. Use disableOverlayClick instead.
  https://github.com/trendmicro-frontend/react-modal/blob/v3.0.0/src/Modal.jsx#L114-L123
• Integrate Webpack 4, Babel 7 and Styleguidist
• Update the code to use the React.createRef() API introduced in React 16.3