Page offset for markdown

[folofjc]

In order to aid in citations, would be nice to have a page offset so that in the combined markdown file, the page has the "real" page numbers and not the pdf page numbers. For a journal article, this would mean that the page offset is the value of the first page (since it is 0 indexed).

Also, this is useful for when a pdf has a copyright page, can include this in the offset so that everything works.

[folofjc]

Should be fixed in #38