Make credentials queries work with npm

Before this change, sl-registry would set `username` in the npm config
but would not provide `password`. This caused npm to complain about
missing credentials. Unfortunately the error message did not make it
clear how to solve the problem. See npm/npm-registry-client#44.

Even when the user figured out he has to run `npm login`, npm would
not ask for a new password, because the username was not being changed.
See npm/npm#5145.

This commits introduces two changes in sl-registry:

 1. `sl-registry use` checks the config option `always-auth`.
  If it is enabled and the npm config does not have all auth
  credentials, `npm login` is executed to let the user authenticate.

 2. `sl-registry add` does not ask for `username`. This way
  `npm login` detects a new username and asks for a password.