remark in a CDN

Serving from GitHub seems to work okay for most people, so I'm closing off this one for now.

@gnab Hi!

I would like you to add support cdn.

Now, we can use https://gnab.github.io/remark/downloads/remark-0.13.min.js or http://remarkjs.com/downloads/remark-0.13.min.js as a external resource.
However, remarkjs.com seems not to have support for https, and github don't support cros.

I cannot find any solutions. If we don't have any solutions, we should reopen this issue.
https://cdnjs.com/ support seems very easy for me. If you say ok, I will request in cdnjs and hope your help when there are troubles uploading this library in cdnjs.

• ref: https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity

remarkjs.com is now available over HTTPS, but remark has also been added to cdnjs via https://github.com/cdnjs/cdnjs/issues/10062