It seems that some time in the last couple years wkhtmltopdf's default
margins were changed from '10mm' to zero. As an alternative to #177,
this PR adds an option to pass in arbitrary wkhtmltopdf long arguments
and sets the default top and bottom margin to '10mm'.
Fix output file name glitch, writing CSS files to destination folder,
and an issue where the process would evaporate before PDF/PNG generation
could complete.