Collapsible areas in list

Issue Summary

I've set up a rather large hierarchical vocabulary and taxonomy_list is a great way to show the structure. But because it's a large vocabulary, the page is pretty long. I'd like to have nested collapsiblility for the branches of the tree.

I was hoping collapsiblility would be triggered simply by adding CSS classes "collapsible collapsed" to taxonomy-list-children or taxonomy-list-table but that didn't work. Clearly some slightly deeper jquery work would do the trick.