Add CommonMark spec examples as tests

[chrisalley]

This is a feature request rather than an issue. It would great to see the CommonMark spec example cases added as tests for better cross compatibility with other Markdown implementations. There are currently 624 examples listed in version 0.28 of the spec.

[am-kantox]

@chrisalley Adding these tests is on my todo list since the very beginning. I will definitely do when I have it robust enough to become a release candidate. I never announced it till today, since I am positive many tests from CommonMark will still fail.

Secondly, before adding tests I need to standardize the default syntax to be 100% in correspondence with markdown, which is not the case now (e.g. ![alt](img) generates a figure tag rather than raw img etc.)

But I do surely agree that before RC it should pass these tests. The thing is I did not decide yet whether I indeed want to be fully compatible with markdown. My main goal is to provide a way to handle flexible easily customizable markdown-like syntax on the fly in the first place.

[chrisalley]

That sounds good. As far as I'm aware, there currently isn't a CommonMark parser written in Elixir. Earmark doesn't follow CommonMark and cmark is a C-wrapper.