This new functionality now makes it possible to execute scripts at the start or end of the build process, while also being super simple to expand to add further script stages in the future.
It also supports effortless reuse of scripts for multiple stages, since the scripts are now executed with the "current stage" as their 1st argument, to allow them to easily determine which stage they're running in.
This fixes the issue where someone specifies `fedora-version: latest`, which won't be known until build-time.
I also added a small "welcome" banner to the build log. It's really just there to retain a somewhat contrived use-case example for how to use `get_yaml_string()`, for other programmers who want to extend this in the future.
You can now define your custom repos with the `%FEDORA_VERSION%` variable, to automatically use the correct repo version, so that you never have to maintain their custom URLs again in the future.
You can now easily remove RPMs from your custom image, without having to edit the build.sh script.
This also changes the old "rpms" config key, to "rpm-install", for consistency with the new setting.
