Summary:
We don't currently support platform extensions in asset modules.
This adds supports for it:
```
require('./a.png');
```
Will require 'a.ios.png' if it exists and 'a.png' if it doesn't.
Summary:
@public
1. Default to first class support of popular image formats
2. Add tests to make sure we support other than png
Test Plan:
1. ./runJestTests.sh
2. Add test.png and test.jpg images in the Playground app dir
3. require both images and render then in the playground app
4. they render
