Convert instory.su internal story format into Ink stories.

This crate provides a CLI that takes a story URL, story ID, or an exported .json file, and generates an equivalent Ink story.

Usage examples:

```sh

With story URL

inkstory https://instory.su/story/12345 > story.ink

With a URL that exports JSON

inkstory https://example.com/story.json > story.ink

With a JSON file

inkstory ./story.json > story.ink

With a story ID

inkstory 12345 > story.ink ```

Option --fix-rn replaces all occurences of the two letters "rn" with newlines.