Read configuration from XDG_CONFIG_HOME

[r-darwish]

In addition to ~/.isort.cfg, isort will try to read a configuration file from
$XDG_CONFIG_HOME/isort.cfg

[r-darwish]

Waiting for #782 in order to use the xdg package.

[jdufresne]

appdirs may be a suitable alternative to consider. https://github.com/ActiveState/appdirs

It is well established, cross platform (Windows), and supports all current Pythons.

[r-darwish]

appdirs returns ~/Library/Application Support in macOS instead of $XDG_CONFIG_HOME/isort.cfg. Is that acceptable?

[jdufresne]

I don't use macOS, but I gather that is the typical spot for configuration files. If so, then yeah, that makes sense to me.

[r-darwish]

It's the typical spot for GUI applications. Command line applications such as fish or git still look
for their configuration file in ~/.config in macOS. I'll do this

[timothycrosley]

Looks good! Thank you!