Add proper CLI

Just wanted to do a little change, but it ended up being a little bit more than that. :open_mouth:

- Add separate cli.js
- bin script in `package.json`
- slightly cleaner code that's easier to test

Let me know if you want me to rebase some of the commits.

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Should probably add some tests, but I'll wait and see if you want these changes first.
"^1.0.0" ?