Add functionality for extra opts

[RincewindsHat]

Providing a general configuration file functionality (espcecially for credentials) seems like a good idea.
So a helper for extraopts could be worth the time

[lazyfrosch]

There is https://github.com/spf13/viper to integrate with pflag and possible environment variables...

[martialblog]

@RincewindsHat I think this falls into the same discussion as #65 right? The question would be, how much of the CLI stuff should this lib handle and how much should downstream code do.

Let's have a talk and see where this fits on the roadmap