[add] new notifier/Discord plugin (#2275)
[add] new notifier/Discord plugin (#2275)

* Add new notifier/Discord plugin

* Update flexget/plugins/notifiers/discord.py - removed unused variable

Co-Authored-By: antoinerime <127cecf39c453d08c1d41513ee87273ed053cbd1@student.uclouvain.be>

* Update flexget/plugins/notifiers/discord.py - removed unused max_retries value

Co-Authored-By: antoinerime <127cecf39c453d08c1d41513ee87273ed053cbd1@student.uclouvain.be>

tobinjt/Flexget,JorisDeRieck/Flexget,Danfocus/Flexget,ianstalk/Flexget,tobinjt/Flexget,JorisDeRieck/Flexget,crawln45/Flexget,Danfocus/Flexget,tobinjt/Flexget,ianstalk/Flexget,tobinjt/Flexget,crawln45/Flexget,crawln45/Flexget,Danfocus/Flexget,Flexget/Flexget,crawln45/Flexget,gazpachoking/Flexget,malkavi/Flexget,JorisDeRieck/Flexget,Flexget/Flexget,malkavi/Flexget,gazpachoking/Flexget,ianstalk/Flexget,Flexget/Flexget,malkavi/Flexget,JorisDeRieck/Flexget,Danfocus/Flexget,malkavi/Flexget,Flexget/Flexget