Improve Electron target

[sinedied]

Follow-up of https://github.com/ngx-rocket/generator-ngx-rocket/issues/114.

With initial Electron support coming in v6, I see multiple areas of improvements to the current implementation, using https://github.com/maximegris/angular-electron as ground for ideas 😉

• [ ] Add electron:serve tasks so hot reload works with Electron
• [ ] Use https://github.com/electron-userland/electron-builder to push further packaging integration