Make it so the main, single, and bottom element selectors are configurable at initialization

[dcarrith]

Currently the main and single element selectors are based on the ulId that is passed in as a setting. The bottom element selector is hard coded as a list divider element. But, that kind of limits the use of the lazyloader widget to listviews. If the three selectors used are configurable, then the widget could theoretically be used to lazy load anything.