Harvest package.json for metadata (electron-builder)

https://api.github.com/repos/wireapp/wire-desktop/git/trees/7d1fadad353ba4dadd9a9c9ccc3598fa007002c4?recursive=1 search for `package.json`, would give us

```
"name": "wireinternal",
"productName": "WireInternal",
"description": "Modern communication, full privacy.",
"main": "main.js",
"updateWinUrl": "https://wire-app.wire.com/win/internal/",
"environment": "internal",
"homepage": "https://wire.com",
"license": "GPL-3.0",
"author": "Wire Swiss ",
"private": true,
"repository": {
"type": "git",
"url": "https://github.com/wireapp/wire-desktop.git"
},
```

Perhaps we can get electron-builder to include this file into the AppImage, @develar?
Or even better, electron-builder could use this information to produce an [AppStream metainfo file](https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps).