[WIP/POC] Koji diff enhancements

- Add tag inheritance to Koji diffs (not sure what fedora tags I can add tests for... need to looking into this more)
- Present comparison of koji tag adds, deletes, changes, and unchanged versions.


I really don't like the names of the methods or the interfaces of the methods, this is really to get an idea of what others think.  I'm hoping the specs help illustrate the use cases of wanting to be able to see what rpm versions were added, removed, changed, or unchanged between two koji tags.