Moves the code which executes subcommands into Subcommand:run,
delegating to separate functions for each subcommand. This reduces the
disgustingness of `run::run` a bit, and paves the way for future
refactoring and cleanup.
Move `env::set_current_dir` outside of `Config::from_matches()` and into
`run()`. It's a bit cleaner this way, and sets us up to make the just
process not change its own directory at all, instead explicitly changing
the working directory of its child processes.
- 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.
