Fix links to the spec

All the links the spec were broken. I changed `http://jgm.github.io/CommonMark/spec.html` to `http://spec.commonmark.org/0.13/`. It'd be nice if there was a `http://spec.commonmark.org/latest/` so you don't need to change the README every time a new version (unless there's major changes so the URL fragments aren't valid anymore.

The spec itself has some broken fragment links, e.g. `#image` instead of `#images`. Unfortunately I don't have a good tool that checks fragments so I don't have the full list.