Add support for GFM-style checkbox task list

[adam-p]

This is from a Google Group post.

Github supports an extension to Markdown for creating checkbox task lists. This MD:

- [x] @mentions, #refs, [links](), **formatting**, and <del>tags</del> are supported
- [x] list syntax is required (any unordered or ordered list supported)
- [x] this is a complete item
- [ ] this is an incomplete item

Turns into this:

• [x] @mentions, #refs, [links](), formatting, and tags are supported
• [x] list syntax is required (any unordered or ordered list supported)
• [x] this is a complete item
• [ ] this is an incomplete item

In case this isn't obvious: An important aspect of the Github feature that won't (and can't) be supported is the the updating of the email (etc.) if the checkboxes are clicked. This makes sense for GH issues, not for email.

[adam-p]

This was also requested in #242, and the user wants to use it in Evernote. Unlike email, maybe Evernote will save the checkbox state? (I was thinking that they would be disabled, but maybe not...)

[chabad360]

Would be sooooo appreciated! :+1:

[11rchitwood]

Looks like this and #242 got closed. Are there any plans to implement checkboxes?

Edit: Is this feature in development?

[stephen147]

+1 for this.

Here's some info from here and looks fairly straight forward to implement.

http://blog.winddweb.com/implement-github-like-checkbox

[stephen147]

You can paste this into the raw text:

<input type="checkbox" class="task-list-item-checkbox" disabled checked>Test line 1
<input type="checkbox" class="task-list-item-checkbox" disabled>Test line 2

It displays like so when converted:

Now, to find a way to change: - [] to <input type="checkbox" class="task-list-item-checkbox" disabled checked>

and

- [x] to <input type="checkbox" class="task-list-item-checkbox" disabled>

This does seem to be possible by wrapping html in the css code as per this: https://www.quora.com/How-do-I-write-CSS-within-HTML https://www.w3schools.com/html/html_css.asp

Same link to where the checkbox code is: http://blog.winddweb.com/implement-github-like-checkbox