Output with Line Numbers

[cgerling]

I'm working on elixir-lang/ex_doc#1074 which includes adding line numbers to the source code output. I wasn't able to find any documentation to include those in the output so I tried to do this with CSS (using list-style with counters) but wasn't possible because this library does not distinguish between regular whitespace and new lines.

Before starting to work on a PR to address this I would like to double-check what is the desired approach for supporting this type of use case. I was thinking of adding the distinction between regular whitespace and new lines which leaves the customization to the client but you might prefer following the steps of pygments and embedding the line information directly into the output.
WDYT?