Support a "solid" archive format

[lapo-luchini]

What's your idea?

Some games have got very similar save files, e.g. Terraria keeps a single previous version as .bak (which maybe we could just skip since it is redundant with Ludusavi options for keeping multiple backups, but that's another issue), but using ZIP file format have the files occupy the whole space each time: Using any "solid" compression format, such as .7z or even .tar.xz this would reduce the used space by almost 50%.