Add line highlighting support

[allejo]

The CommonMark spec allows for any arbitrary content to exist in the info string of a code block. Would you be open to adding support for highlighting specific lines of code within said code block?

```html{2}
<div>
  <p>I wish I wish I was a fish</p>
</div>
```

Proposed Syntax

• html{4} - highlight the 4th line of the code block
• html{4-5} - highlight the 4th through 5th lines of code
• html{1,4-5} - highlight the 1st and 4th through 5th line of code separately

[sebastiandedeyne]

I'd very much be interested in this!

I don't really have time to implement this myself atm, but I'll gladly accept a PR.

[allejo]

Sure thing! I can work on a PR for this, just wanted to make sure it'd be received favorably before I worked on it.