Add the ability to add user notes

[rnelson]

At some point forever ago, I added the following into the readme:

Prior to writing KeysExport Viewer, I just tossed generated keys into a text file. The advantage this had over parsing the XML is that I could add notes, documenting things like which VM a specific serial number was used in.

"Someday" I want to build a way to add additional notes like that. Perhaps a zip file (with some custom extension) that contains a SQLite database and the most recently provided XML file?

I'm not tied to any one implementation idea at the moment, but some way to add notes without modifying the input XML file (so you can regenerate it and not lose custom content) would be nice.