HTML span elements only apply to a single line, and in the case of multi-line
comments, the highlighting was dropped as a result. Ensure that each line has
the proper styling to fix this.
Closes#1577
This custom formatter for rouge is needed to generate HTML output
specifically for gitlab. Since its usecase is mostly suitable for
gitlab it had been rejected upstream:
https://github.com/jneen/rouge/pull/268
Thanks a lot to @stanhu, @jneen and @tsigo for review!
