Add GS1 capability

[OtherBarry]

I wanted to use this library for a project, but having to replace the "X" in isbn was annoying, and there was no way to handle GS1 codes other than GTIN-13

I've written tests for GTIN-8, GTIN-12, GTIN-13, GTIN-14, GSIN and SSCC, using the check digit calculator on the GS1 site.

Let me know if there's anything else that needs to be done to get this merged.

[harens]

Wow! Thanks so much @OtherBarry for this PR, it looks amazing 🎉 (and matches the style of the other files eerily well). I'll run the tests, and assuming they pass, I'm more than happy to merge this.

I wanted to use this library for a project, but having to replace the "X" in isbn was annoying

Thank you for using this project. I'll push a minor release with your work so that you can use this.

[harens]

Thanks @OtherBarry for your PR! 👍

I'll publish a new release sometime today. Now that I know other people are using this project, I'll try and spend some more time refactoring and adding new features.

Thanks again for your pull request! 🍾

[harens]

Thanks again for your amazing work @OtherBarry! @all-contributors please add @OtherBarry for code. 🎉

[allcontributors]

@harens

I've put up a pull request to add @OtherBarry! 🎉

[harens]

Actually, as well as that, @all-contributors please add @OtherBarry for test.

[allcontributors]

@harens

I've put up a pull request to add @OtherBarry! 🎉