title: Setting platform dependant icon via electron-forge electronPackagerConfig
body: <p>Using electron-forge to build a desktop app. The app is built for both OSX &amp; Windows. Inside my package.json, I have: </p>
  
  <pre><code>"electronPackagerConfig": {
      "icon": "src/images/icon/app_icon_osx.icns"
  }
  </code></pre>
  
  <p>When I build on Windows, I'm having to manually change the icon in package.json from "app_icon_osx.icns" to "app_icon_win.ico". </p>
  
  <p>If I try to just use "app_icon.png" for both platforms, the icon doesn't show on OSX, and the build fails with "rcedit.exe failed with exit code 1. Fatal error: Unable to set icon" on Windows.</p>
  
  <p>I have all 3 versions of the icon (icns, ico, png) in a folder in the project. Because i'm using electron-forge, i don't seem to be able to use electron packager's --icon argument.</p>
  
  <p>Is there a way I can pass the icon to use as a command line arg, instead of hardcoded in package.json? Or a way I can use the same png for both platforms?</p>