refactor: Vars::cli_arg returns Result<&str>

[dsgallups]

This is a small QOL adjustment that allows you to quickly pattern match on a CLI argument. i.e.

match vars.cli_arg("foo")? {
  "apple" => ...
  "banana" => ...
  _ => ...
}

Sometimes when the value is optional...

match vars.cli_arg("foo") {
  Ok( "apple") => ...
  Ok(_) => ...
  Err(e) => ...
}

I don't see any particular behavior that would be lost from auto-dereffing the String here. It seems that &str is more often more useful than &String.