Install plugin only in a specific platform (defined in package.json)

# Feature Request

## Motivation Behind Feature

Many plugins do work only in a specific platform and crash in other platforms during build. Furthermore many plugins are specifically designed to a specific platform only, but their creators did not take that into account. There's also a popular [request](https://stackoverflow.com/questions/23108342/install-cordova-plugin-for-one-platform-only) for this in Stackoverflow.

## Feature Description

It would be very advantageous to have in `package.json` some sort of way to inform that we just want a specific plugin installed only in a specific platform, and not in all platforms.

For example something like this in `package.json`

```
"cordova": {
"plugins": {
"cordova-plugin-mobile-ocr": {
"platform-specific": ["android"]
}
}
```

## Alternatives or Workarounds

We can use plugman but it's not reliable nor there is a way to tell other people that we want a certain plugin installed only on a specific platform. I suppose we can also use a hook, but I was wondering before which event should we consider to process. We can also have git branches with different versions of `package.json` for different platforms.

The best way would be undoubtedly to have that information directly stored in `package.json`.

Start by reviewing the package.json cordova.plugins configuration shown in the issue, then compare the plugman and hook workarounds it mentions. Determine how platform-specific plugin installation should be represented and processed; done means a plugin declared for one platform is not installed for other platforms without breaking existing package.json behavior.

Written by the indexing model from the issue text.