Support XDG_HOME

[rgbkrk]

One glaring omission in jupyter-paths is all the XDG_* handling and Xdg directories in general. Simple to implement, just needs doing.

Reference in Jupyter code base:

https://github.com/jupyter/jupyter_core/blob/38548d6b1004978591e6e492389a361ff686ac4a/jupyter_core/paths.py#L91-L94