Allow the filename of the PDF to be configured

Setting @filename in the action allows you to specify the name the file should be downloaded as, should a use try to save the PDF.

I'm not sure why, but it seems to be common to have `call` return a string, otherwise I don't believe the `controller` is available. I looked to [csv_shaper](https://github.com/paulspringett/csv_shaper/blob/45d06d3ecca845e8d9ef90b04a94ade12ae3196d/lib/csv_shaper_template.rb#L25-L37) for reference as they have a similar feature and that's how they handle it.