Make npm registry url configurable

Just came across one of our client projects, which uses their own npm registry instead of the official one. By making it configurable via cli-flag like --registry https://my-registry.com we could support that use case.

I'm wondering if this is how we can make our npm plugin testable too! We could spin up a fake registry since we only ever download the tarball and use the --registry flag in our tests to point to our mocked server.