allow custom tarball url

Just one more thing: I want people to be able to specify a custom tarball url, since otherwise clients will just get "http://localhost:5080" even when someone has set up `local-npm` behind a proxy, in order for it to be used by multiple people.
so the reason I'm letting them do the full url prefix is that I cannot assume they want http vs. https, or what hostname they want, or what port they want, assuming they're using a reverse proxy or something