[2.x] Add support for highlighting/focusing in CommonMark

[ryangjchandler]

```php {2,4-8}{3}
// ...
\```

This is now supported out of the box in the CommonMark extension.

The first set of braces does highlighting. The second does focusing.

Highlighted lines get a highlight class added. Focused lines get a focus class.

You can pass a plain integer 2 or a range of integers 4-8, comma-separating each value.