This commit introduces support for using `embark.config.js` to calculate the
embark configuration object that is otherwise provided via `embark.json`.
If an `embark.config.js` file is present, it will be used over the
`embark.json` file. The `embark.config.js` module needs to export either an
object or a function that can be asynchronous and has to return or resolve with
an embark configuration object:
```js
// embark.config.js
module.exports = async function () {
let config = ...; // do lazy calculation of `embarkConfig`;
return config;
}
```
Michael Bradley, Jr