subliminal: add authentication option
subliminal: add authentication option

Some subtitle providers require authentication. To configure them,
subliminal accepts the `provider_configs` option to the ProviderPool
class. Add support for it through an `authentication` configuration key.

Having the provider configurations be specified in the `providers` key
would look nicer, but break the case where one wants to authenticate to
specific providers, but still use all the available ones (by leaving out
the `providers` option completely).

jacobmetrick/Flexget,JorisDeRieck/Flexget,dsemi/Flexget,oxc/Flexget,ianstalk/Flexget,Flexget/Flexget,sean797/Flexget,malkavi/Flexget,LynxyssCZ/Flexget,jacobmetrick/Flexget,qk4l/Flexget,qk4l/Flexget,Danfocus/Flexget,crawln45/Flexget,jacobmetrick/Flexget,poulpito/Flexget,JorisDeRieck/Flexget,LynxyssCZ/Flexget,crawln45/Flexget,oxc/Flexget,OmgOhnoes/Flexget,tarzasai/Flexget,ianstalk/Flexget,gazpachoking/Flexget,OmgOhnoes/Flexget,Danfocus/Flexget,oxc/Flexget,malkavi/Flexget,crawln45/Flexget,tobinjt/Flexget,jawilson/Flexget,malkavi/Flexget,Danfocus/Flexget,tobinjt/Flexget,jawilson/Flexget,crawln45/Flexget,drwyrm/Flexget,sean797/Flexget,dsemi/Flexget,jawilson/Flexget,sean797/Flexget,gazpachoking/Flexget,Danfocus/Flexget,drwyrm/Flexget,JorisDeRieck/Flexget,poulpito/Flexget,LynxyssCZ/Flexget,JorisDeRieck/Flexget,poulpito/Flexget,tobinjt/Flexget,ianstalk/Flexget,Flexget/Flexget,jawilson/Flexget,tarzasai/Flexget,LynxyssCZ/Flexget,drwyrm/Flexget,Flexget/Flexget,Flexget/Flexget,qk4l/Flexget,dsemi/Flexget,tarzasai/Flexget,malkavi/Flexget,OmgOhnoes/Flexget,tobinjt/Flexget