As we all know upsolving is very important to improve your level because it forces you to solve harder problems which you couldn't solve during contest. I wrote a python code which uses codeforces API to find all the unsolved problems. You can find more description in the link. I know it's a bit slow(takes 4-6 seconds to parse one participated contest). That's why it saves the list in a file at the end. Please give it a try. :)
Known issue — This fetches almost all problems. Not all because http://codeforces.net/api/problemset.problems doesn't return all contest problems as expected. :( Need to find a fix for this.. Do send a pull request to github repo if you find it first. ;)
What about sorting by difficulty as well? Probably it's not hard to add this:)
Yes indeed! I have added this to a separate file. Now this doesn't have any bugs because it doesn't depend https://codeforces.net/api/problemset.problems
Does difficulty of problem relate to solved count?