Prettify: converted to using single init element

1. Plugin now uses a `.wb-prettify` element as initialization
   point.  All settings are configured from CSS classes and/or
   data attributes on this element.
2. Updated to use new 4.0 plugin template.
Prettify: updated demos

1. Updated to match change to how plugin now uses a single
   `.wb-prettify` element to initialize.
2. Ported French demo from v3.1.