Add more specs, strikethrough / task list improvements

[nobodywasishere]

• gfm-spec.txt was missing tests that are available on the website (see Published HTML version of specification is out of date with spec.txt github/cmark-gfm#288), so instead I've added some other specs that were available on the gfm repo
• gfm-extra.txt was removed as it was no longer needed
• Minor improvements to strikethrough / task lists to pass new specs

With this PR, there are 49 pending specs remaining, primarily focused around table / autolink / footnote support.

[nobodywasishere]

I'm also happy to throw my hat into the ring as a maintainer, and do think this project should be moved under the crystal-lang org (if possible).

[icyleaf]

Review crystal-lang/crystal#4613 (comment) please

[nobodywasishere]

Review crystal-lang/crystal#4613 (comment) please

Ah yeah, disregard my latter comment. It probably won't happen anytime soon (especially given it hasn't happened in 6 years...)

[icyleaf]

If you are willing, I can add you to the project team. Considering that the crystal project is dependent on this project, handing it directly over to someone else is not a wise choice. What do you think?

[nobodywasishere]

That's completely fine with me. Have similar thoughts about handing the project wholesale to someone else. I'll probably keep with using PRs and ask people in the community to review them so it's not completely on you.

[icyleaf]

invitation was sent