Allow environment variable expansion in config

[NickLarsenNZ]

Feature Description

I would like to use the same config (in my dotfiles repo) across machines, but the savePath is not the same between them.

Eg:

[General]
savePath=${PICTURES_PATH} ; for example

That way I could set the variable for each machine without having to change the file and have a dirty git branch for the config.

[focusaurus]

Also some way to deal with linux vs mac home directory path with $HOME or ~ would be nice.