Support Evernote task lists.

[makethemo]

This feature is only available for Evernote. Now we can create Evernote task lists with the GFM-style checkbox. It also supports saving checkbox state. To selectively apply this feature, I added the code for Options.

GFM-style checkbox:

- [ ] have to do
- [x] something

Evernote task lists:

Evernote task lists are represented by the following code:

<div>
  <div>
    <img class="en-todo" src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"> have to do
  </div>
  <div>
    <img class="en-todo en-todo-checked" src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"> something
  </div>
</div>