CLI

I'd like to do:

```shell
$ npx medium-to-markdown ericclemmons
```

or

```
$ npx medium-to-markdown https://medium.com/@ericclemmons/javascript-fatigue-48d4011b6fc4?format=json
```

Start by reviewing the two requested invocations: `npx medium-to-markdown ericclemmons` and the full Medium URL with `?format=json`. Run them to establish current behavior, then locate the CLI entry point that handles arguments. Done means both forms accept their input and convert the requested Medium content to Markdown.

Written by the indexing model from the issue text.