use viper for standardised configuration (12 factor)

[doodlesbykumbi]

consider using https://github.com/spf13/cobra for the CLI and https://github.com/spf13/viper.

viper would DRY out some of the 12-factor configuration implementation

[kgilpin]

This makes sense to me for the top-level CLI flags (e.g. -debug), but not for the full configuration of listeners and handlers. Because I see Viper being written to handle simple flags rather than complex configuration objects.