Allow third-party extensions of Netlify CMS as separate packages.

[austincondiff]

These plugins could be installed and updated via npm instead of having our own plugin repo. We could update in the UI and update the package.json, then refresh the page once the deploy is complete.

Third party plugins could open things up to additional widgets, dashboard sections, services such as Netlify/Google analytics or even entire collections. Unlike other CMS's, these plugins would not introduce vulnerabilities in your site as they will not run on the front-end site. They will only add additional functionality to Netlify CMS.