Add pdf output format

When using the notebook for generating graphics for my SIAM poster, I found that I was making and remaking pdfs through the latex output. It seems like it might be handy to be able to tell nbconvert to output pdf directly rather than having to separately run pdflatex (yes I know it is one command, and I will not be offended if you say to just run it separately). 

    nbconvert -f pdf notebook.ipynb

I have put together a simple proof of concept implementation that chains a call to pdflatex on after generating the .tex. Is this something that would be a welcome addition? If it is I can add some more robust error handling and any other desired cleanup.