feat(splash-screen): add config option for dark mode background color

[kendyl93]

I've added a new backgroundColorDarkMode value to allow you to edit the splash screen background color with dark mode enabled on your device. With the changes below you will be able to edit the background color directly from the capacitor.config.json file, for example:

{
  "appId": "someID",
  "appName": "some name",
  "bundledWebRuntime": false,
  "npmClient": "npm",
  "webDir": "www",
  "cordova": {},
  "plugins": {
    "SplashScreen": {
      "backgroundColor": "#ffffff",
      "backgroundColorDarkMode": "#000000"
    }
  }
}

[imhoffd]

@kendyl93 We may wait until 1.0.0 of the new splash screen plugin is released to include this (which will happen about when Capacitor 3 is released). Thanks for your patience!