Improve npm package support in Playground

[vtrifonov]

Currently when you add an npm package it is downloaded in a folder under the app folder with the name of the package and to be able to use it you need to reference the package by providing relative url to its folder.
It will be good if all those packages go inside a packages or node_modules folder which can be styled differently than the other folders and try to find a way to reference the package by giving just the package name.
In addition to this when you download the project that folder should not be included in the archive, but those packages should be added inside the package.json dependencies.