Follow XDG dirs if set on macos

[dmmalam]

If XDG_CONFIG_HOME, or XDG_DATA_HOME are set, then use those dirs for config, and history, just like on linux. The current dirs are technically correct for macos, but many command lines tools prefer these if set, and it keeps a nice separation between gui/ and cross platform cmd apps. Also tools like pearcleaner flag the files as 'orphaned' as they can't match it to an installed App/.

[sharkdp]

Related: dirs-dev/directories-rs#47. We would probably need to switch to https://docs.rs/etcetera/latest/etcetera/index.html to fix this.