Option to set page.paperSize for PDF capture

When generating a PDF, need to be able to set generated paper size.

I can't find where this is documented for PhantomJS, but the implementation is here https://github.com/ariya/phantomjs/pull/15 and it's used in rasterize.js https://github.com/ariya/phantomjs/blob/master/examples/rasterize.js#L15