Add support for XDG base directory specification

[luka-hash]

The spec: https://specifications.freedesktop.org/basedir/latest/

Practical difference - Tigris' config/data would not be stored in ~/.tigris/, but in (e.g.) ~/.config/tigris/, or depending on the type of the data, ~/.local/share/tigris/. Other platforms have their own path defined by the specification, i.e. this is not Linux-specific.

I could also work on this if you are open for PRs.

[designcode]

@luka-hash thanks for pointing out. Feel free to open a PR