chore: refactor config file setup #46

The global config file and the flow config files work completely different from one another.
I think we can refactor both to use Viper under the hood.
Combining Viper with acorn-io/cmd (based on Cobra), we should come to way easier terms especially with the global config file that is a mixture of flag-configurable options and non-flag-configurable options.
Also this should allow us to make all flags configurable via config file as well - this may need an extension of acorn-io/cmd