Rollup plugin

Develop a Rollup plugin to resolve platform cascades/extensions.

This could be accompanied by a Webpack plugin, for web entrypoints, but extension resolving for apps could be avoided by using this code pattern:

**`core.js`**

```js
let _module;

if (typeof window !== 'undefined) {
_module = require('./core.web');
} else {
_module = require('./polyfill'); // JS/React only implementation e.g. render null + console.warns
}

module.exports = _module;
```