Feature: flag to print parsed options (--show-options)

[fvsch]

Implement a --show-options (or --show-config) CLI flag that prints resolved options to stdout.

Rationale: one goal of this tool is to make every feature configurable through CLI options. This requires custom formats and parsing rules. I'm trying to print warnings and errors for obviously malformed inputs, but some issues can go undetected, and letting users check what options are actually resolving to could be useful.

Alternative: a more generic approach, with a DEBUG=true or --debug flag that prints resolved options but also more detailed information at all stages.