Use standardized config directories for storing `config.yaml`

[rsubtil]

The current behavior of reading a config file from the current workdir presents a few problems, particularly on Linux:

• It doesn't allow launching the app from a different path. This makes it harder for scripting and/or autostart behavior, as it requires cding into the executable directory (https://github.com/omriharel/deej/issues/48)
• The executable directory might not be writable, making configuration changes difficult. This makes packaging this app for various distros difficult, as modifying any packaged files is generally not advised.
• Due to the above, this app is not suitable to be available on typical paths (e.g. /bin, /usr/bin)

While these problems are centered on Linux, I'd suggest following standard practices for config file placement between different OSes as a robust way to fix such issues. https://github.com/kirsle/configdir is a cross-platform library that can handle this, and I've patched my version of deej to use this and it works without any issue. I'd contribute these changes, but since you're not accepting PRs, the best I can is to notify you of this friction on adoption on more Linux distros :slightly_smiling_face: