CLI: improve plugins help messages, refactor code
CLI: improve plugins help messages, refactor code

mrb/letsencrypt,dietsche/letsencrypt,lbeltrame/letsencrypt,VladimirTyrin/letsencrypt,thanatos/lets-encrypt-preview,rugk/letsencrypt,jsha/letsencrypt,modulexcite/letsencrypt,jsha/letsencrypt,Sveder/letsencrypt,BKreisel/letsencrypt,Sveder/letsencrypt,ruo91/letsencrypt,TheBoegl/letsencrypt,tyagi-prashant/letsencrypt,jmaurice/letsencrypt,rlustin/letsencrypt,mrb/letsencrypt,vcavallo/letsencrypt,rutsky/letsencrypt,rugk/letsencrypt,goofwear/letsencrypt,ghyde/letsencrypt,bsmr-misc-forks/letsencrypt,Bachmann1234/letsencrypt,Hasimir/letsencrypt,tdfischer/lets-encrypt-preview,dietsche/letsencrypt,sapics/letsencrypt,Hasimir/letsencrypt,piru/letsencrypt,Jadaw1n/letsencrypt,ruo91/letsencrypt,jtl999/certbot,bsmr-misc-forks/letsencrypt,TheBoegl/letsencrypt,kuba/letsencrypt,bestwpw/letsencrypt,ahojjati/letsencrypt,hsduk/lets-encrypt-preview,kuba/letsencrypt,tyagi-prashant/letsencrypt,sjerdo/letsencrypt,beermix/letsencrypt,stweil/letsencrypt,tdfischer/lets-encrypt-preview,Bachmann1234/letsencrypt,goofwear/letsencrypt,rlustin/letsencrypt,luorenjin/letsencrypt,letsencrypt/letsencrypt,ahojjati/letsencrypt,jmhodges/letsencrypt,sjerdo/letsencrypt,hsduk/lets-encrypt-preview,lbeltrame/letsencrypt,PeterMosmans/letsencrypt,deserted/letsencrypt,jtl999/certbot,piru/letsencrypt,fmarier/letsencrypt,sapics/letsencrypt,brentdax/letsencrypt,martindale/letsencrypt,hlieberman/letsencrypt,twstrike/le_for_patching,PeterMosmans/letsencrypt,deserted/letsencrypt,kevinlondon/letsencrypt,BillKeenan/lets-encrypt-preview,fmarier/letsencrypt,stewnorriss/letsencrypt,luorenjin/letsencrypt,BillKeenan/lets-encrypt-preview,hlieberman/letsencrypt,stweil/letsencrypt,beermix/letsencrypt,martindale/letsencrypt,rutsky/letsencrypt,lmcro/letsencrypt,vcavallo/letsencrypt,DavidGarciaCat/letsencrypt,Jonadabe/letsencrypt,riseofthetigers/letsencrypt,armersong/letsencrypt,lmcro/letsencrypt,solidgoldbomb/letsencrypt,xgin/letsencrypt,g1franc/lets-encrypt-preview,brentdax/letsencrypt,kevinlondon/letsencrypt,skynet/letsencrypt,wteiken/letsencrypt,DavidGarciaCat/letsencrypt,mitnk/letsencrypt,wteiken/letsencrypt,xgin/letsencrypt,skynet/letsencrypt,riseofthetigers/letsencrypt,stewnorriss/letsencrypt,jmhodges/letsencrypt,Jonadabe/letsencrypt,BKreisel/letsencrypt,armersong/letsencrypt,thanatos/lets-encrypt-preview,ghyde/letsencrypt,Jadaw1n/letsencrypt,bestwpw/letsencrypt,solidgoldbomb/letsencrypt,twstrike/le_for_patching,g1franc/lets-encrypt-preview,modulexcite/letsencrypt,VladimirTyrin/letsencrypt,mitnk/letsencrypt,jmaurice/letsencrypt,letsencrypt/letsencrypt