feat(emacs/lean-input): add not(¬) and iff(↔)
feat(emacs/lean-input): add not(¬) and iff(↔)

close leanprover/tutorial#81

leanprover/lean,levnach/lean,javra/lean,leanprover-community/lean,Kha/lean,eigengrau/lean,dselsam/lean,digama0/lean,avigad/lean,avigad/lean,sp3ctum/lean,fpvandoorn/lean2,Kha/lean,fpvandoorn/lean,johoelzl/lean,johoelzl/lean,leanprover-community/lean,avigad/lean,sp3ctum/lean,soonhokong/lean,soonhokong/lean-windows,UlrikBuchholtz/lean,fgdorais/lean,soonhokong/lean,levnach/lean,leanprover-community/lean,c-cube/lean,leanprover-community/lean,c-cube/lean,fpvandoorn/lean,soonhokong/lean-windows,fpvandoorn/lean,leanprover-community/lean,javra/lean,rlewis1988/lean,dselsam/lean,fpvandoorn/lean,leodemoura/lean,soonhokong/lean-osx,fpvandoorn/lean2,rlewis1988/lean,c-cube/lean,rlewis1988/lean,Kha/lean,soonhokong/lean-osx,digama0/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover-community/lean,htzh/lean,soonhokong/lean,fpvandoorn/lean2,soonhokong/lean,leodemoura/lean,rlewis1988/lean,johoelzl/lean,c-cube/lean,UlrikBuchholtz/lean,dselsam/lean,leodemoura/lean,eigengrau/lean,fgdorais/lean,UlrikBuchholtz/lean,c-cube/lean,leanprover/lean,levnach/lean,rlewis1988/lean,htzh/lean,levnach/lean,javra/lean,dselsam/lean,soonhokong/lean-osx,eigengrau/lean,fpvandoorn/lean2,soonhokong/lean,soonhokong/lean-windows,Kha/lean,soonhokong/lean-windows,johoelzl/lean,rlewis1988/lean,UlrikBuchholtz/lean,leanprover/lean,fgdorais/lean,UlrikBuchholtz/lean,dselsam/lean,digama0/lean,dselsam/lean,htzh/lean,Kha/lean,eigengrau/lean,johoelzl/lean,avigad/lean,soonhokong/lean-osx,digama0/lean,fgdorais/lean,sp3ctum/lean,htzh/lean,digama0/lean,leodemoura/lean,avigad/lean,leanprover/lean,leodemoura/lean,javra/lean,soonhokong/lean-osx,soonhokong/lean-windows,digama0/lean,sp3ctum/lean,sp3ctum/lean,eigengrau/lean,johoelzl/lean,Kha/lean,fpvandoorn/lean2,avigad/lean,htzh/lean,leanprover/lean,leanprover/lean,fgdorais/lean,javra/lean,levnach/lean,fgdorais/lean,leodemoura/lean