feat(js/main_live.js): remember the last opened chapter and load it
feat(js/main_live.js): remember the last opened chapter and load it

close #49

avigad/tutorial,avigad/tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,soonhokong/lean-tutorial,soonhokong/lean-tutorial,avigad/tutorial,Hodapp87/cs6460_project,Hodapp87/cs6460_project,cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,leanprover/tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,fpvandoorn/tutorial,leanprover/tutorial,soonhokong/lean-tutorial