Add support for private registries

[sventschui]

Add support for .npmrc and private registries.

1. API in

   wmr/src/plugins/npm-plugin/registry.js

   Line 54 in 71b80e0

   const API = 'https://registry.npmjs.org';

   should not be hardcoded but rather resolved by looking for a .npmrc file up in the tree and resolve the registry from there. This should also support registries specified for certain scopes. Related to Make npm registry url configurable #167
2. Add support for authenticated registries. Use credentials from .npmrc or ask for it in interactive mode

See https://github.com/yarnpkg/yarn/blob/a4708b29ac74df97bac45365cba4f1d62537ceb7/src/registries/npm-registry.js for some inspiration

Edit: A "simple" solution could be to run npm config list --json and parse its JSON output. It will give the registry config in the registry and @<scope>:registry keys. Also it provides userconfig and globalconfig keys that can be used to try to lookup auth tokens in these files. Also <registry_url>:always-auth keys should be respected to send credentials when trying to resolve packages.

[marvinhagemeister]

Let's track this in #167 👍