Making `highlight` behave more like redcarpet

See issue #2510 for details.

Note: redcarpet does not escape the class name (like Jekyll does).

This means that `c++` in redcarpet generates `class="language-c++"`, while in Jekyll will be `c--`. Should I also remove this `gsub("+", "-")` call, so that both generate the same code? And why is it here anyway?