Change default save state location from "~/.smex-items" to "~/.cache/smexitems"

[truesilver92]

Assuming it is a cache that keeps track of your historically selected commands I think it should be moved to ~/.cache/ so that it matches with the xdg standard (I think), more easily gets excluded from backups of the home directory, and reduces clutter in the home directory.

What do you think?