Added option to add blank pages to support duplex printing

[mdriessen]

Hello! Thank you for this library, it is very useful for merging PDF's.

Some of the documents I merge are printed double sided and I was having the problem that some attachments where printing on the back of the letter. So I added a duplex option when adding files. When this option is set to true a blank page is inserted to ensure the document starts at an even page count.

Thanks!