feat: configuration file support

[cmdoret]

It should be possible to provide configuration options via a config file, for options which are largely invariant within a project / environment. We should follow general recommendations (clig.dev, 12 factor).

NOTE: adding a -c/--config flag will require renaming the rules specification flag (maybe -r/--rules?)

Objective: support layered config via cli options or config flags

Requirements:

• decide on implementation (possible solutions: config-rs, clap+serde)
• rename existing options as needed
• implement according to above recommendations
• demo config + tests

[cmdoret]

Started something with clap+serde in the branch feat/config-file, but this would require more work because:

• optional variables do not work: as missing arguments cause clap to fail
• merging config with args is not possible as-is
  This would require a more advanced crate (clap-serde or something simlar), which requires defining a separate config struct.
  Maybe overkill for now