This supports client-certificates that some secure servers require to establish a secure connection.
"cert" is the client's certificate, and "key" is the private key for that certificate. These are merely passed through from npm-registry-client's user to npmLogin the same way the "ca" parameter is.
add support for client cert/key, needed by https/tls
Pass-through cert and key to couchLogin to support https/tls client certificate features needed when npm connects to a registry that requires client certificates.
adding documentation for cert and key options
add optional support for client cert/key, used by https/tls
passing through from config to https request