add pagenumber parameter

add pagenumber parameter

this allows picking a page from a pdf. also, this enhances performance
greatly when rendering pdfs, as only the first page is rasterized.
(otherwise, imagemagick would treat the pdf as a list of images, work
with all of them, until finally only the first page gets saved). the
default parameter of 0 will select the single image contained in typical
image files anyway, so no specialcasing between single- and multifile
containers is needed.