link check, max_articles, processed tag

* Continues despite bad links e.g. 404 errors, using a try/except block
  and testing each link

* `max_articles` option to define how many pinboard articles to pull at
  most

* `processed_tag` option--a tag added to any post pulled. This way you
  can find all the links that have been processed once the `toread` flag
  is no longer set.
Updated readme

- Updated the readme to describe the new `max_articles` and
  `processed_tag` options.