Allow passing extra args to code highlighter

[yuvipanda]

I primarily want to allow for the possibility of line numbers
in HTML output. Pygments supports this natively, but we don't
have a way to pass arguments through to pygments.

A traitlet is added to allow passing arbitrary extra args
to the Pygments formatter, for Html and LaTeX. Should allow for
line number control and much more :)

This has been something folks have asked for for a while.

Ref ipython/ipython#5847
Ref #427
Ref notebook-sharing-space/nbss#34

[yuvipanda]

Have to actually test this though.

[yuvipanda]

I tested it and it works! All hail traitlets!

jupyter nbconvert a.ipynb --to html --Highlight2HTML.extra_formatter_options linenos=table

[yuvipanda]

And I can call it from code like this:

    traitlets_config = {
        "Highlight2HTML": {"extra_formatter_options": {"linenos": "table"}}
    }
    exporter = HTMLExporter(
        # Input / output prompts are empty left gutter space
        # Let's remove them. If we want gutters, we can CSS them.
        exclude_input_prompt=True,
        exclude_output_prompt=True,
        extra_template_basedirs=[BASE_PATH],
        template_name="nbconvert-template",
        config=traitlets_config,
    )

Ready for review I think.

[SylvainCorlay]

This looks good to me. Approving the PR.

[yuvipanda]

Thanks a lot, @SylvainCorlay!