Some fixes

I realized that parent level <li> should be closed in function start_el, instead of end_el.
That is because off canvas menu list is not nested.
So you can just discard my former pull_request, and add blank end_el function.
This time, I overwritten end_el to close only child(second) level <li>.
Whichever would work I guess.

Another commit I made is to add some attributes to menu item's <a>, like target="_blank".