Allow skipping saved window size

[stuartlangridge]

I'm seeing this issue as well. I'm currently always removing the config/cache file in the script that I use to open/close Magnus. I'm using Devilspie2 to automatically set the window size and position, so I don't need the window size to be remembered. Originally posted by @kankaristo in https://github.com/stuartlangridge/magnus/issues/10#issuecomment-739874921

I propose to fix this by adding a new command line option to ignore any saved size.