sexy-bash-prompt: Exit code reporting

In #80 we will work out how to support opt-in switches for extension. Exit code reporting is planned. For now we can discuss here what it should look like.

I’ve seen this feature in a few different shell prompts over the years. There are different ways of doing it. A couple things we could try, off the top:

1. A special STATUS_SUCCESS and STATUS_FAILED color that modifies an existing token (like the prompt character [$])
2. The same colors that modify a new token, also user-configurable