Tasklist items don't handle nested sublists correctly

[NightFlyer]

There are bugs in the tasklist extension that cause tasklist items to mess up when handling nested sublists. Specifically, if the sublist is indented 4 spaces rather than 2, the list structure is incorrect. So, this code:

- [x] foo
    - [ ] bar
    - [x] baz
- [ ] bim

produces a list structure of:

<ul>
  <li><input type="checkbox" checked="" disabled="" /> foo
        <ul>
           <li><input type="checkbox" disabled="" /> bar
                 <ul>
                     <li><input type="checkbox" checked="" disabled="" /> baz</li>
                 </ul>
          </li>
      </ul>
 </li>
 <li><input type="checkbox" disabled="" /> bim</li>
</ul>

Notice how baz has been made a sublist of bar. However, if you use the following
code (same as before, but replacing the space or x with @ means that it isn't recognized
as a tasklist):

- [@] foo
    - [@] bar
    - [@] baz
- [@] bim

you get the correct list structure of:

<ul>
  <li>[@] foo
    <ul>
      <li>[@] bar</li>
      <li>[@] baz</li>
    </ul>
  </li>
  <li>[@] bim</li>
</ul>

[kivikakk]

Thank you for this detailed report, and the attendant fix!