patched extensions/codehilite.py to allow global line number override

Waylan,

I took the time to add this functionality while leaving the existing functionality intact for backwards compatibility.  Please take a look at `output.html` and `original_output.html` (generated by `test_patch/test.py`).

If you have any questions about what I did or why I did it, let me know.

Cheers,
noah