add tasklist extension

[Watson1978]

This patch will parse Task list items (extension) in GitHub Flavored Markdown Spec.

$ cat sample1.md
- foo
  - [ ] bar
    - baz
      - [x] hoge
          - [ ] fuga

$ ./cmark-gfm -e tasklist ~/sample1.md
<ul>
<li>foo
<ul>
<li class='task-list-item'><input type='checkbox' disabled='' />bar
<ul>
<li>baz
<ul>
<li class='task-list-item'><input type='checkbox' checked='' disabled='' />hoge
<ul>
<li class='task-list-item'><input type='checkbox' disabled='' />fuga</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>

[eldridge]

This extension is working well for me on Linux. It looks like the AppVeyor build is failing because compiler warnings are treated as errors and the two unused variables in the no-op _scan_tasklist function are generating warnings.

One suggestion I have is to include a element for the task item content so that replacement of the input form element can be done with pure CSS a la this StackOverflow thread.

[waldyrious]

Hey @Watson1978, out of curiosity -- would supporting tri-state checkboxes be in-scope for this extension? The lack of an indeterminate or N/A state has been a consistent point of friction for me in GitHub's current implementation.

[doits]

Any chance this gets picked up? It is the only thing missing for me to switch to this library for markdown rendering.

[kivikakk]

It makes sense. I'll test this and merge.

[kivikakk]

Added some tests and addressed bugs that showed as a result.

[kivikakk]

@Watson1978 Let me know what name you'd like to be used in attribution for this PR for the changelog and releases!

[kivikakk]

Likewise, let me know if you need a new release made.

[Watson1978]

@kivikakk Thank you for merging this!! I prefer that you will use my github account name :)

[doits]

Any chance to have a new version released because of this?

[kivikakk]

@doits I've tagged the release 0.28.3.gfm.20!

[doits]

@kivikakk thanks!