Extra arguments to pandoc

I made this change locally at one point to allow extra arguments to be passed to `pandoc`.

I'm not sure if this change will be useful to anyone else, and I'm no longer even using this feature myself. But here's how I was using it at the time: `judo myfile --template "david" --pandocargs "--self-contained"`

Anyway, certainly no offense taken if you don't think it's worth merging this, but I wanted to push it in case you do, and of course, thanks again for Judo, Gadly, etc.