Allow using the XDG dirs on macOS.

[porglezomp]

Often command line tools on macOS use unix locations for dotfiles, I essentially never look in ~/Library/ to configure my tools. Because of this, I've avoided this crate for my personal projects because I want my dotfiles in ~/.config. Being able to configure this would make this much more useful to me.

[andybarron]

I'm planning on a rewrite that would let you select a preset rather than being forced to always use platform defaults.

[thegedge]

We'd love this for nushell (see nushell/nushell#893), as we believe Library isn't the best place for data of a command-line only app (given how Apple documents some of the subdirectories, hinting towards app bundles).

@andybarron Any updates here? Anything we can help with?