Check links for errors, add a new tag to processed links, and max # articles

Thanks for writing Pinboard-Recipe! It's great!

I have a backlog of like 5 years' worth of links that are marked `toread`, and the recipe was overwhelmed by the number as well as some of the links, which were no longer available and made Calibre error out after `toread` had already been unset.

This code tests the links before sending them to Calibre, and also adds options to pull only a certain number of links and to add a new tag so that you can find the links you've processed.