Add XDG support

[luis-guimaraes-exoawk]

Hey, it'd be great if the Contabo CLI could respect the XDG Base Directory Specification

Thanks

[tahergalal]

Can I ask what are the benifits of the XDG Base directory?

[luis-guimaraes-exoawk]

@tahergalal It's a standard, it helps create a cleaner more organized directory structure, and makes the behavior consistent between applications.
This makes it so that most users won't have to fight around with Contabo's config file and/or lose time reading about the configuration file and the way it overrides it's options.