BREAKING: Refactor script execution summary

[jmgate]

I'd like the script execution summary to be more customizable. I'd like the base class to provide a variety of SummaryItems (for lack of a better name). Subclass developers can extend/override what's provided by the base class, and they can create their own. When they call print_script_execution_summary(), they can pass it a list of summary items for it to process, or it can fall back to the default list provided by the base class.

Note: This will likely be a breaking change, though it might be possible to provide a backward compatibility layer for a time.