Add a hook before setting the highlighted code to <code>'s innerHTML

Adding a hook after the code is highlighted and before it's added to `<code>`, can be useful, If you want to change the highlighted code before it's added to html.

In my case, I wanted to split the lines of highlighted code and wrap a `<span>` around each line before setting it to innerHTML. I tried `Prism.highlight()`, but using `highlightElement()` is easier and faster(with async enabled) in my case.

So I added a hook to Prism and I'm using it. I thought I suggest it here, perhaps it can be useful to others, too.