Integrate configuration layer with CLI

The configuration layer is currently not considered when running the repod-file script.

When looking at !40 (closed) it becomes clear though, that it would be good to start integrating it, so that the script can gain functionality such as -r/ --repo (setting the target repo, defined in a configuration).

For this to be useful on a system (think repod running as a service) vs. user level (e.g. just a user running repod-file locally) it might be useful to distinguish between system/user mode. In system mode we could make use of some of the system-wide defaults (https://gitlab.archlinux.org/archlinux/repod/-/blob/79a0fa9aa4a4d9def6dad24805f9bbff0388f734/repod/config/defaults.py), in user mode, we could default to e.g. well-defined (e.g. XDG based) directory structures to store repos by default.