Robert Crumb is known for his disturbing, yet compelling, underground cartoons: his most famous works made countercultural icons out of Mr. Natural ("Keep on Truckin'...") and Fritz the Cat. Terry Zwigoff delves into the odd world of the cartoonist in his documentary film Crumb, and the picture that emerges is not always pretty--at moments, it's almost repellent--but it's a fascinating glimpse …