Toyota showed off a special version of its new Mirai saloon this week to celebrate Back to the Future day. The Japanese carmaker’s take on the time-travelling DeLorean from the popular movie series has the trademark gull-wing doors and the same, bare-metal bodywork seen in the Hollywood film. But Toyota’s car is not fuelled by banana skins. This one runs on hydrogen. Mirai — meaning “future” in Japanese — is Toyota’s fuel-cell vehicle, a hybrid…