Installing private packages that are not hosted on the public npmjs registry

Modules are currently tied to the public npmjs registry. The search bar searches the public registry to add modules. There is no way to side-load modules into the fiddle.

Ideally there would be a `package.json`-esque way to control the npm resolution process.

If that is not desirable, one workaround is to specify paths to tarballs downloaded with `npm pack $PACKAGE_NAME`