Last week’s movie conversation was all about a certain very high-priced vanity documentary with a ginormous promotional budget. But that attention-magnet did not win the weekend box office, nor did “Shelter,” the crown going to a second heavily-advertised mainstream thriller, “Send Help.” However, it just narrowly beat out a new release that was on almost no one’s radar — except, it turns out, a huge online fanbase.  “ Iron Lung ” is a first theatrical feature endeavor for Mark Fischbach aka