Support for Single Size iOS Image Icons

### :speech_balloon: Description

Xcode supports both `All Sizes` and `Single Size` as options for an App Icon asset. It would be great to update this tool to generate the Single Size option as well as it reduces the number of Assets bundled with the app.

For context, the `Content.json` is also much simpler, requiring just one entry like so:
```
{
"images" : [
{
"filename" : "AppIcon--1024x1024@1x.png",
"idiom" : "universal",
"platform" : "ios",
"size" : "1024x1024"
}
],
"info" : {
"author" : "xcode",
"version" : 1
}
}

```