Disable npm user config

[pikajude]

A user's ~/.npmrc might have, for example, a different registry URL, which could end up generating the wrong package set. This behavior can be disabled manually with npm_config_userconfig=/dev/null npm2nix [...], but it'd be nicer not to need to remember.

[cstrahan]

We probably ought to default it to /dev/null, but provide a flag to choose the config. My team makes use of an internal registry of private packages, so we'd need a way to use configure registries and SSL options.

[pikajude]

I think everyone's team uses a registry of private packages, but on the other hand, I don't believe fetchurl can handle private registries yet...