Define a simple in-memory key-value store for our cache.
TL;DR:
- Define `Cache` as a simple state-keeping `Agent`
- Define `Sup`, which starts our Cache process
- Define `App`, which starts our Supervisor process
- Whitelist `App` in `mix.exs`, so that it starts after calling `iex -S mix`
In case I want to package this project with Nix. For now, it's useful to store
this at the project root because it help my Emacs's `project-find-file`
function.
