Provide the ability to specify more specific image assets

### SUMMARY

Currently images are output as
* Android:
* `drawable-hdpi`
* `drawable-mdpi`
* `drawable-xhdpi`
* `drawable-xxhdpi`
* `drawable-xxxhdpi`
* iOS
* `"idiom" : "universal"` inside `Contents.json`

Propose that more complex scenarios can be catered for differentiating between iPad and iPhone screens (e.g. `idiom` in Contents.json) and the more complex android naming for resources (e.g. `drawable-sw600dp`). I am still trying to hunt down the Android link for the valuable detail on this.

### DETAILS

I am still trying to work out how this might work. I was assuming it could slot in to the `additionalOutputs` option within a specific platform entry in an image.json file, e.g.

```json
{
"$schema": "http://mobilebuildtools.com/schemas/v2/resourceDefinition.schema.json",
"name": "logo",
"scale": 1.0,
"ios": {
"additionalOutputs": [
{
"name": "logo",
"idiom": "ipad"
}
]
}
}
```

### PLATFORMS

- [x] Android
- [x] iOS
- [ ] UWP