Allows for a inside li elements in tab panels

Previously, all `ul` elements at any level inside the tab container were activated. This fix activates only direct children `ul` elements of the tab container. This allows for `ul`s containing `a` elements inside the tab container without breaking the tabs.

I ran a quick web-based minification to generate the minified version. If you have a preferred minification method, I'll be glad to use that instead.