Receive viper instance for configuration

[budak7273]

Suggested by @mircearoata

[mircearoata]

This would allow other code using this as a library to use viper for its config without sharing the key names.

For example, one could configure ficsit-cli in viper.Sub("ficsit") and pass that viper instance to ficsit-cli, and then use viper.Sub("my-app") for its own config that can be saved/loaded from config files separately.