Ability to mark subtree in render() for pre-rendering

[KevinGrandon]

Hello! @chrislloyd and myself here from Pinterest. We're currently looking at ways of doing grid layout in React and have a need to pre-render elements to calculate positioning data. We're contemplating using an approach similar to this pull request, and were soliciting feedback on how something like this would look in the core.

I'm worried about performance issues with the current implementation of preRender(), defined inside "02 - Layout Components". The main problem I currently see is that the component itself may have to do additional work to format children before they are measured. As another method you might need to split out your JSX from render(), and have that return an array of formatted objects. Being able to mark a section of the subtree to pre-render, as in this approach, feels cleaner.

Thanks!