Add support for babel-plugin-flow-react-proptypes

The problem

react-docgen isn't very reliable, and doesn't work at all with styled components.

Proposed solution

Allow to generate the documentation from React PropTypes autogenerated with babel-plugin-flow-react-proptypes starting from the Flow types.

Alternative solutions

Fix react-docgen, but it's not trivial.

Additional context

I think it's a low hanging fruit that will improve the quality of the generated documentation for Flow users. It may even be used as a fallback solution if react-docgen fails.