Create or verify a user named <username> in the specified registry, and
save the credentials to the .npmrc file. If no registry is specified,
the default registry will be used (see config).
The username, password, and email are read in from prompts.
You may use this command multiple times with the same user account to
authorize on a new machine. When authenticating on a new machine,
the username, password and email address must all match with
your existing record.
npm login is an alias to adduser and behaves exactly the same way.