Support npm authentication

[davideicardi]

Npm registry that require authentication need to pass the params object to the npm-registry-client. Params should be:

{
auth: {
                alwaysAuth: true,
                token: "your-token"
            }
}

Provide a way to pass this, maybe using the npm config already available?

Under the hood it just add an header: headers.authorization = 'Bearer ' + credentials.token