[React] Add support for arrays with holes in style attribute

Summary:
See the style proposal on react-future: https://github.com/reactjs/react-future/blob/master/04%20-%20Layout/Inline%20Style%20Extension.md

The goal is to enable writing CSS purely in JavaScript. We've been using this for a couple of internal projects and it's proven to be a very expressive tool.

Note that this diff doesn't bring `StyleSheet.create` nor `StylePropType`. I'm keeping it simple by just allowing arbitrary arrays, objects and falsy values.

Test Plan:

```
grunt test
```

Reviewers: jwalke zpao sema
CC: tomocchino