* Add built-in support for --version flag
* Test that command-defined --version overrides the built-in.
* Document the `version:` parameter in CommandConfiguration
* Include --version in the generated help.
* Add an API for converting an error to an exit code
* Make ExitCode more useful as a value type
* Update tests to use ExitCode values
* Typo fix
* Add a test for ExitCode.isSuccess
* Switch to just using ExitCode for tests