[add] Add a threshold setting next_series_seasons
[add] Add a threshold setting next_series_seasons

* Add a threshold setting for number of episodes already downloaded to next_series_seasons

* Use default config value for threshold

qk4l/Flexget,OmgOhnoes/Flexget,malkavi/Flexget,OmgOhnoes/Flexget,Danfocus/Flexget,Flexget/Flexget,malkavi/Flexget,ianstalk/Flexget,JorisDeRieck/Flexget,jawilson/Flexget,jawilson/Flexget,LynxyssCZ/Flexget,OmgOhnoes/Flexget,JorisDeRieck/Flexget,LynxyssCZ/Flexget,tobinjt/Flexget,crawln45/Flexget,Danfocus/Flexget,LynxyssCZ/Flexget,Flexget/Flexget,tobinjt/Flexget,jawilson/Flexget,crawln45/Flexget,gazpachoking/Flexget,ianstalk/Flexget,ianstalk/Flexget,crawln45/Flexget,jawilson/Flexget,malkavi/Flexget,Flexget/Flexget,malkavi/Flexget,tobinjt/Flexget,tobinjt/Flexget,Danfocus/Flexget,gazpachoking/Flexget,JorisDeRieck/Flexget,qk4l/Flexget,qk4l/Flexget,Flexget/Flexget,JorisDeRieck/Flexget,crawln45/Flexget,Danfocus/Flexget,LynxyssCZ/Flexget