`env_var(key)` looks up the value of the environment variable with name `key`, aborting execution if it is not found.
`env_var_or_default(key, default)` looks up the value of the environment variable with name `key`, returning `default` if it is not found.
- Switch to asciidoc, since it supports an auto-generated table of contents
- Re-organize into sections
- Document private recipes
- Document windows dependencies
- Document doc comments
