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.
Eventually, there will probably be a `crate` visibility specifier that
does the same thing as `pub(crate)`. This commit replaces `pub` with
`pub(crate)`, so when `crate` is available we can easily switch to it.
Just's dependency on brev was the cause of a
fairly deep branch of the transitive dependency
tree. To decrease build time and make the life of
packagers easier, this diff moves the functionality
that Just was using in Brev into Just itself, and
removes the dependency on Brev.
Fortunately, the only functionality that Just was
using was the output function and OutputError
enum, so this was easily done.
`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.
