Better support for custom types

[chrisui]

This is more of a "would you be willing to accept a pull request which does this?" question than an issue of sorts. :)

I would really like to extend the functionality of "custom" types so that:

1. We support destructured imports (Eg. PropTypes.array - note the lack of React.)
2. We allow common react-esque prop type function patterns to be picked up with custom props in the same way that arrayOf is currently handled. (Eg. PropTypes.iterableOf(PropTypes.record(User)))

Basically I feel the library should be able to support custom user prop types in such a way we don't have to reparse the raw value which would mean anyone can easily implement simple custom prop types and create documentation from that.

Custom prop types are super useful in building a real-world app with React and so I would like to see how we can help developers document these too.

We, at Lystable, want to build an open living component docs solution and are hoping react-docgen can feature at the core!