move data to XDG dir instead of $HOME

[shapr]

FLM should be using https://hackage.haskell.org/package/directory-1.3.5.0/docs/System-Directory.html#v:getXdgDirectory instead of getHomeDirectory when deciding where to put user data.

[shapr]

This issue needs a change at https://github.com/shapr/fermatslastmargin/blob/master/Main.hs#L34