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.

[aleex42]

Very good idea - maybe specify user/pass OR cert/key ...

[pbirokas]

@andrazkopac when you have already ceated an solution for authentication with certificates, can you please provide an Pull request?
I am not familiar with perl, that makes it hard for me to implement such a change by myself.
Thanks in advance