* Large refactor with lots of features added:
** Added `render` command to allow renders of prepared markdown letters
** Added `output-directory` config option to specify where letters should be saved
** Added an internal backup directory to store intermediate versions (and prevent accidental deletes!)
** Added ability to inspect config even if it is invalid
** Added ability for user to provide their own prompts instead of the builtins
** Added an option for the user to provide an editor command
** Improved LLM prompts for `generate` command