Move injection of @new tag tooltip to Process::Versions.

This class is doing with the processing of @new and @since tags already,
so it might as well take care of this @new related task.

This gets rid of the special #options= method in Tag::New and
TagRegistry.
Inject the assets to @aside tag directly.

Don't pass them through the TagRegistry.  Either way it's a hack, but
doing it through TagRegistry is only adding an extra layer of
indirection.