Request: Support Private Scoped Modules

[shellscape]

Referencing sindresorhus/latest-version#3, it would be great for this module to support private modules. This probably requires using the npm module rather than the HTTPS endpoint in latest-version. Assuming that won't happen, it may be a good option to allow consumers of update-notifier to specify which 'latest' module they'd like to use. Users in my situation could pass a reference to the latest module, which uses npm and the user's .npmrc with the auth token to access private modules.