Issue filtering for get-pdfs-of-issues.py

[willnewton]

This adds a few options to get-pdfs-of-issues.py to filter what issues are handled and how much data is printed.

[mhl]

I think this can be reduced to:

            if not any(l['name'] == label for l in issue['labels']):
                continue

[mhl]

Thanks very much for the pull request! This all looks useful - I left a small comment where I think the label code could be simplified.

My only other thought was that it might be nice to use the API for fetching an issue label and a milestone to check that the user's supplied a label / milestone that actually exists before iterating over all the issues. I'm happy to merge this without that, though, if you don't think that's worth changing this PR for :)