Support $XDG_DATA_HOME and $XDG_CONFIG_HOME

[nbeaver]

nvpy currently defaults to storing config in ~/.nvpy.cfg and notes in ~/.nvpy/.

https://github.com/cpbotha/nvpy/blob/2039574f98b927cc883dd251bcc90200d5adf137/nvpy/nvpy.py#L148

It would be tremendously cromulent if nvpy would default to storing config in $XDG_CONFIG_HOME (usually ~/.config/) and notes in $XDG_DATA_HOME (usually ~/.local/share).

There is a helper library called python-xdg, more details here:

https://askubuntu.com/questions/457047/how-can-i-get-the-xdg-default-user-directories-from-python

but if minimizing dependencies is desirable, it would be straightforward to implement the relevant parts of the specification. The FreeDesktop spec is here:

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html