Added page header and footer option; Resolves #12

I've added a parameter to pass in a CommonJS (for PhantomJS, you know...) module that sets the pages' header and footer properties as described [here](https://github.com/ariya/phantomjs/wiki/API-Reference-WebPage#wiki-webpage-paperSize). By that it is possible to create headers and footers which can contain the page number. Therefore a solution for #12.