[Markdown plugin] add line number offset for code sections to markdown

[flowolf]

added the option for a line number offset.

this was already possible with data-ln-number-offset now it's possible in markdown as well.
see https://revealjs.com/code/#line-number-offset-4.2.0

I've openend a PR for revealjs.com too: reveal/revealjs.com#53

What changed

the option to add line highlights is extended by an optional number offset for line numbers:

```javascript [1-5] 
or 
```javascript [256: 1-5]
or 
```javascript [256:]

are valid.

[hakimel]

Nice addition, thanks @flowolf!

[flowolf]

thanks for an awesome project! happy to contribute, @hakimel!

[mattcen]

Closes #3386