Feature idea: code highlighting

[giltho]

Code highlighting is particularly helpful when you want to show some specific feature in the context of broader code. Take for instance this example in the Stripe documentation which shows an entire file with the relevant bit highlighted.

I'm not 100% sure how to achieve that in HTML, but possibly from the point of view of odoc it should look like the following to highlight lines 4 to 6:

{@ocaml highlight="4:6" [
...
]}