output: add a basic output package
Created by: LawnGnome
This gives us some basic primitives to output structured, human readable information from commands. This is currently intended only for the campaigns work taking place in #260, but also gives us options for additional functionality elsewhere, such as internal/api.
This is somewhat duplicative with things that we currently have in cmd/src and internal/campaigns; I intend to gradually clean those up as time permits.
This will be included in #260 as well, but I've split it out for easier review. Here's a demo: