Docsplit images command - Added the ability to specify the page number delimiter via a command line option

When using docsplit images to split a pdf into a series of images (e.g. foo.pdf --> foo_1.jpg, foo_2.jpg, ...), the user may want the ability to specify the page number delimiter. For example, instead of always using an underscore ('_'), the user may want to use a dash ('-') or something more complicated (e.g. foo.pdf --> foo-page-1.pdf, foo-page-2, ...).