Add the ability to track multi-part episodes with custom_mappings

[cantnamethis]

Currently it's not possible to track multi-episode anime with the script, because either Anilist or TVDB would have either too many or too few episodes respectively, a fix would be adding "multiplier" and "divider" variables to the season entries.

Example for Panty & Stocking with Garterbelt where there are 26 episodes on TVDB but only 13 on Anilist

entries:
  - title: "Panty & Stocking with Garterbelt"
    seasons:
      - season: 1
        anilist-id: 8795
        divider: 2

This would update Anilist once for every 2 episodes watched on Plex, the same could be done in reverse with the multiplier option,
an example with The Disastrous Life of Saiki K. where there are 120 on Anilist, but only 25 on TVDB

entries:
  - title: "The Disastrous Life of Saiki K."
    seasons:
      - season: 1
        anilist-id: 21804
        multiplier: 5

Now when 1 episode is watched on Plex, the script updates 5 on Anilist.

[Soitora]

I thought at first this was something where they split the seasons, but you're right. It counts episodes differently.
I think an example where this would work is if you installed the Re:Zero Season 1 Directors Cut, in which you'd want this "multiplier" feature

[Ravencentric]

This is a really great idea and would be awesome if implemented.