Add reached-start and reached-end CustomEvents#65

DialBird wants to merge 3 commits intojohnfactotum:mainjohnfactotum/foliate-js:mainfrom

DialBird:feature/add_at_end_and_start_to_fixedDialBird/foliate-js:feature/add_at_end_and_start_to_fixedCopy head branch name to clipboard