Add ngx-scripts support for Electron

[sinedied]

Related: https://github.com/ngx-rocket/generator-ngx-rocket/issues/418

Current Electron build is far from perfect:

• it requires separate command to build / run / package app
• running app in serve mode is tedious: 2 separate commands to run in parallel + 1 to run the app
• packaged apps includes a not of unwanted files, bloating the resulting package

Some of these issues are related to the usage of electron-packager and could be resolved by changing to electron-builder with appropriate configuration as in #418 (the bloating), but the others would be simpler to tacker with a specific support for Electron in ngx-scripts, like how it's done for Cordova support.

Basically, it would be nice to mimic the Cordova workflow with something like 3 base scripts: electron:run, electron:build and electron:clean with additional params to enable live-reload (--serve), skip rebuild (--fast) or target a specific platform (windows|mac|linux) for the build. The plumbing would then be done under the hood 😉