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

[reviewher]

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