scrollFirstHighlightIntoView = function() {
					Plugin.scrollHighlightedLineIntoView( block, scrollState, true );
					slide.removeEventListener( 'visible', scrollFirstHighlightIntoView );
				}
Highlights a code block. If the <code> node has the
'data-line-numbers' attribute we also generate slide
numbers.

If the block contains multiple line highlight steps,
we clone the block and create a fragment for each step.