Added `checked` class to checked li elements.
Added `checked` class to checked li elements.

ilyavf/multi-select,icanjs/multi-select,marshallswain/multi-select,ilyavf/multi-select,ilyavf/multi-select,marshallswain/multi-select,marshallswain/multi-select,icanjs/multi-select,icanjs/multi-select