- Differentiate between `arg`s, which are flags and options, and `cmd`s,
which are mutually exclusive subcommands
- Replace string literals, like "EVALUATE", with constants, like
`cmd::EVALUATE`, since they're slightly less error prone.
- Remove `Config::evaluate`, and handle it like other subcommands
run::run() is pretty unwieldy. As a first step in improving it, this
commit pulls most of the argument parsing into the `config` module.
It also renames `Configuration` to `Config`, just to be easier to type.
