API: Allow and/or prefer to use a standalone file for configuration

[lessp]

As to separate the logic and not clutter up esy-configurations I propose that we either allow for a separate config-file to be used, or refactor to only use a configuration-file.

E.g.

revery-packager.config.json

{
   "bundleName":"ExampleApp",
   "bundleId":"com.example.app",
   "displayName":"Revery Example App",
   "mainExecutable":"App",
   "windows":{
      "packages":[
         "zip"
      ],
      "iconFile":"assets/icon.ico"
   },
   "darwin":{
      "packages":[
         "tar",
         "dmg"
      ],
      "iconFile":"assets/icon.icns"
   },
   "linux":{
      "packages":[
         "tar",
         "appimage"
      ]
   }
}