use index instead of range ref
Sort downloaded documents after downloading

These are downloaded out of order so they need to be reordered by their
position in the original array after everything is finished.
Recreate Command object before every invocation
Call by value
Fix page offsetting of nested bookmarks

page offset should be zero-indexed from the parent document, not offset
by one.
Generate full combined bookmark output

This generates the input to `cpdf -add-bookmarks` on the combined
document.
Write bookmarks to combined document