Add option to minify manifest

[ai]

Right now we server manifest in a readable format:

{
  "icons": [
    {
      "src": "/icon_512x512.16e41258d4647f0e3f5a235e566e8cbf.png",
      "sizes": "512x512",
      "type": "image/png"
    },
    {
      "src": "/icon_192x192.9fbf91e65dde2dd06f534bd7781cbc69.png",
      "sizes": "192x192",
      "type": "image/png"
    }
  ],
  "name": "Akeero",
  "short_name": "Akeero",
  "orientation": "portrait",
  "display": "standalone",
  "start_url": ".",
  "background_color": "#363636",
  "theme_color": "#363636"
}

Why not add a minification for production to save the Internet a little.