Support global config directory in ProjectDirs

[pheki]

Hi! It would be great if this library supported global config directories... My idea is:

Platform	Value	Example
Linux	/etc/_project_path_	/etc/barapp
macOS	/etc/_project_path_ or /usr/local/etc/_project_path_ (if installed via homebrew)	/etc/barapp
Windows	%ProgramData%\_project_path_\config	C:\ProgramData\Bar App\config

MacOS implementation challenges:

• Only the "bundle_id" is passed to project_dirs_from_path, which makes it hard to get the _project_path_.
• How to detect if running through homebrew? Does homebrew patch repositories to change /etc/ to /usr/local/etc?

Another idea would be to use /Library/Application Support instead...

Kind of related to #78

[ModProg]

One thing to add here is that the actual xdg spec uses XDG_CONFIG_DIRS which is /etc/xdg by default.

[andrewbaxter]

These are all related: #87 #78 #58