LaTeX writer:  Support line numbering with `--listings`.

If "numberLines" class is present, we add "numbers=left";
if "startFrom" is present, we add "firstnumber=".

Partially addresses #763.
LaTeX reader:  Read optional attributes in lstlisting environment.

We convert these to pandoc standard names, e.g. "numberLines"
for "numbers=left", "startFrom=100" from "firstnumber=100".

Still need to add code to convert the language names.
Updated changelog.

It still needs a lot of cleaning up before a release.