PARIS — Honda took the cover off its Insight hybrid concept car today here at the Paris Motor Show, and with it took direct aim at the new Toyota Prius. As Toyota officials work to electrify their champion of the hybrid market, Honda was hoping to take the leap from niche to mainstream by delivering a relatively conventional, four-door, five-seat subcompact sedan at what Honda expects will be the lowest price tag of any hybrid-electric vehicle on the market.