Support for hidden selector; block links, and templates.

* "-hidden" hides a particular element only when in that specific state. Useful for toggling between an active/inactive UI.
* Common style for block links - class="block": display with useful hover differentiation and styling.
* Templates: if you give any element in your prototype an ID attribute, it can be used as a template. Use data-clone on any other div to dump the contents of the template inside it. Recursive use of this feature is not supported or caught, so don't do it.