Support XDG Base Directory specification

[fenuks]

Currently, brainworkshop on Unixes keeps data in '~/.brainworkshop` which is problematic, because it clutters home directory with dotfiles, and there is no separation between configuration, generated files like logs, etc. XDG Base Directory specification lists several places where user data could be placed, I think that article on Arch Wiki summarizes the idea pretty well. It would be great if brainworkshop could follow the specification as well.