Add an option to get a specific bookmared spec

[davidanthoff]

Would be great if getBookmarkedSpecs had an integer option so that one could say "give me the 3rd bookmarked spec". https://github.com/davidanthoff/DataVoyager.jl provides an option for users to extract a specific bookmarked spec from a voyager instance from code, and right now there is quite a bit of overhead involved because it always transfers all bookmarked specs into the julia process, even if the user only asked for one.