Highlight Focus code line style by default or plugin

[AbraaoAlves]

It would be amazing if there were a way to display code using syntax like this

``` js@focus=3:6,14

It'll be possible if we can simply add HighlightJS plugin like it: https://github.com/huytd/highlightjs-focus

[RDhar]

This would be great to have, and one of the reasons that reveal.js is still used due to its highlight-lines bracket syntax.