Allow required packages to specify their own transform rules

[jxe]

Browserify has a hackish way to do this, with a section inside package.json.

What would be ideal would be a pre-processor independent way of saying what is being required inside a module. E.g.,

# HappyReactComponent/package.json
{
   "codebase": "es6 jsx png myth"
}

Or maybe it should use mime types or something.