Exclude a book cover from Kobo's screensaver

For books without real or with a bad cover, or books you don't want others to see that you are reading them.

No global variables were created with this CR! ;-)
The source code is around the beginning of docsettings.lua. Just read the open and readSetting methods. You should be find this out in one second with 'ack' or 'ag' command. If you are not using it, I strongly suggest you give it a try. I use it everyday for my work and after working hacking.