Add parameter for nbconvert configuration options

[tdpetrou]

With nbconvert, you have the option of specifying a large number of configuration options. We even set a single config option here with PDFExporter. Perhaps we can add a config parameter, have it accept a dictionary, and forward it to PDFExporter.

All of the preprocessors and exporters have config options, so I think it will be messy to try and parse all of the options. Maybe we just do PDFExporter options first?

I needed this personally to use a different latex template.