It does seem more appropriate to have a separate module for line numbers. I would definitely understand the separation of concerns argument, but the maintainer instead makes some sort of emotional appeal about clutter and simplicity to explain why there is no line number support.
> the maintainer instead makes some sort of emotional appeal about clutter and simplicity to explain why there is no line number support
Clutter and simplicity is not an emotional argument. It becomes verifiably more difficult to maintain a code base with each new line. And since he's the one doing the maintaining, his position seems quite reasonable.
Secondly, he seems to have a strong aesthetic aversion to line numbers, and I can totally respect that. Frankly, among popular open source projects, those whose developers have the strongest sense of style and design and usually (not always) the best.
Many popular syntax highlighting libraries do include this functionality. For example, the popular Python syntax highlighter "Pygments" supports line numbers[0].
