GFM: Strikethrough support

[nobodywasishere]

This pull request adds GitHub Flavored Markdown (GFM) strikethrough support (~~hello~~ => hello).

This also adds the gfm-spec.txt downloaded from here. To prevent a lot of broken specs that won't be fixed by this PR, the strikethrough specs have been copied to gfm-extra.txt temporarily.

require "./src/markd"

markdown = <<-MD
# Hello Markd

~~parser~~
:abc:

> Yet another markdown ~~parser :abc:~~ built for speed, written in Crystal, Compliant to CommonMark specification.
MD

options = Markd::Options.new(gfm: true, emoji: true)
puts Markd.to_html(markdown, options)

<h1>Hello Markd</h1>
<p><del>parser</del>
🔤</p>
<blockquote>
<p>Yet another markdown <del>parser 🔤</del> built for speed, written in Crystal, Compliant to CommonMark specification.</p>
</blockquote>

[robacarp]

This is great. I left one nitpicky comment. Thank you for your strategic approach to this.

[robacarp]

I appreciate your strategic approach here, thank you.

My only suggestion is that this file be named something other than "gfm-extra". How about "working-gfm-spec.txt" or something similar?

[nobodywasishere]

That would work for me. I also wanted a file to add GFM specs for stuff that wasn't covered by the normal spec (like leaving ~ if there's only one set around a char). I can split these into two different files as well.

[nobodywasishere]

Actually, I think it'd be better to leave gfm-spec.txt as-is and instead of bringing pieces over to a new file piecemeal, to mark things in gfm-spec that don't/shouldn't pass as pending. Thoughts?

[nobodywasishere]

Went with the latter as there are currently only ~36 failing tests in gfm-spec.txt

[icyleaf]

LGTM