Add tasklist extra

[liguangsheng]

Add tasklist extra

See Issue #216

[nicholasserra]

Thanks for the PR :)

Looks like this is failing a bunch of tests. Run make testone on your machine to run the test suite. We also only accept patches with test coverage. Please reference other PRs to see an example of a test being added.

[nicholasserra]

Replaced with #218

[cosven]

Thanks for the contribution for the task_list extra, but I have different thoughts on the task-list-wrapper-text.

Currently, the task-list-wrapper-text is:
_task_list_warpper_str = r'<p><input type="checkbox" class="task-list-item-checkbox" %sdisabled>%s</p>'. In my opinion, the <p> tag is unnecessary.

I think the better option will be:
<input type="checkbox" class="task-list-item-checkbox" disabled>%s, as github also handles in this way.

I also have a simple comment on that PR: https://github.com/trentm/python-markdown2/pull/218/files#r100302342

@liguangsheng @nicholasserra

[cosven]

I add a PR here: #243