Collect contextTypes the same way as propTypes

[andyearnshaw]

I'm hoping to get support for this in BlueKit, but it relies on react-docgen to collect the props. I'm developing a library that uses context to pass theme (and some other props) through to descendant components.

contextTypes work the same way as propTypes, so it should be possible to reuse the code.

Edit: looks like there's a dead PR for this already, #130.

[danez]

This will be in 3.0.0