Add authentication with certificate

[andrazkopac]

Security policies prevent using username and password in some companies. API is able to authenticate with certificate. We should enable this option here, too. It could be a minimal change or proper implementation. I was able to do a minimum change by adding parameter --cert, which then uses the same --username and --password parameters for the public certificate filepath and the private key filepath. This is not a complete implementation but is a quick and simple fix that works for us. I can code something better too, but please suggest how it should be implemented to comply best to standards in this project.