Adds support for --env.config argument to Nativescript CLI commands.
--env.config <name> will load $ProjectDir/config/<name>.json and inject it as $ProjectDir/app/config.json
If config contains an "app_name" key, the native app will have its product name updated.
The config file is injected into the app as "config.json", so that it can be loaded at runtime.
tns plugin add nativescript-dev-appconfigtns prepare android --env.config beta will load $ProjectDir/config/beta.json
If beta.json contains "app_name": "My Beta App",
it will update product name in Info.plist for iOS and strings.xml for Android.