Support using a config file in ~/.config if no config file is given on the command line

[jwodder]

This would be a breaking change to apply, as its command-line usage would have to take the config file as an option rather than a non-option argument.

See https://stackoverflow.com/a/76341487/744178 regarding using a dynamic path (likely from dirs or similar) as the default config file path.

On the other hand, the whole motivation for this issue is so that I can run labelmaker without having to cd into the directory where I store my config file, but I'd still need a place to store my list of labelmaker-managed repositories, and moving the list to the config file after implementing #19 would just make it harder to automatically regenerate the list (unless I use toml_edit to preserve the formatting...), so maybe I'm doomed to always cd somewhere anyway.