emphasize-lines should be re-aligned to use remote line numbers

[Starlight220]

Currently it uses line numbers relative to the snippet's content, meaning I have to do this:

      :lines: 298-328
      :linenos:
      :lineno-start: 298
      :emphasize-lines: 15,20-21

rather than

      :lines: 298-328
      :linenos:
      :lineno-start: 298
      :emphasize-lines: 313,318-319

[sciencewhiz]

I agree this would be nice, but it also matches sphinx's literalinclude, so I'm not sure we'd want to change it unless sphinx does