Consistent NA handling

[richfitz]

Most functions should take vectors of paths, and most of these should allow for fairly consistent NA treatment. For now I'm leaving comments in the source and will see what the common features are. But supporting the standard "omit", "fail", "exclude", etc seems like a good idea. Common utility functions like na_screen should help keep things consistent. Python doesn't have to deal with this so we're on our own here.