URL regex fails for valid URL

[Barabazs]

The current regex fails to validate 'https://hackmd.io/@abdelhamid/ethereum-alignment' as a valid URL.

I would suggest to replace the regex with a library like https://validators.readthedocs.io or fix the regex pattern. It might also be a good idea to add a --source=url type of option in the CLI.

PS: I can create a PR if you agree.

[GjjvdBurg]

Thanks for reporting this @Barabazs! Both of those sound like great ideas, and I'd welcome your PR!