Use XDG_CONFIG_HOME on Linux for config file

[txtsd]

Here is the freedesktop spec.

Basically, when installing as a system package on various distros, all the files go in /usr/share/ or /opt/. These directories aren't the correct location to place a config file in. It would ideally go in $XDG_CONFIG_HOME/yt-spammer-purge/config.json so that the program can read it no matter where it's run from.

It doesn't have to read only from this directory, there can be a prioritized list of directories that it will try to read from.
e.g.: Check if file exists in pwd first, then check $XDG_CONFIG_HOME, then look for a default in $XDG_CONFIG_DIRS

This can be extended to Windows too, which uses %APPDATA%\Roaming to store configs and such.

There is a python package called platformdirs that can make this easy to handle, but it isn't necessary.