Support series offset in missing episode provider

Also includes a few other changes to better support anime. 

One of these could potentially cause performance issues (O(n^2) dictionary lookups over series), although in practise you would probably need quite a massive library for this cause issues. I think I could improve that, but it would further complicate the code.

It will also now search through later seasons looking for an episode if the episode number larger than the number of episodes in the season.