Refactor CLI options parsing

[hpjansson]

The CLI options parsing in chafa.c has grown into a big, unwieldy and hard to follow blob. It also has all kinds of ad-hoc logic (defaults, dependent settings, etc) mixed in. We need to refactor this, preferably into its own options.c unit that can produce a struct with just the raw options.

A couple of points:

• Every option must have an auto or unset state.
• It should be possible to merge option structs from different sources in the future (e.g. from config file, env vars).
• Defaults must be applied by separate logic after the options have been parsed and merged. Consider that some of the defaults may come from probing (which itself can be turned off with an option).