Nick Offerman, in addition to being a goddamned national treasure, has, like the H. Jon Benjamins of the world, a voice that makes everything better, funnier, more beautiful, etc. That’s a quality that filmmaker Eric Kissack exploits to its fullest in “The Gunfighter,” a short comedy that features Offerman in the role of an omniscient narrator turning up the tension in a classic Western “man walks into a saloon” scene.