Breaking Change
Instead of requiring a Vec<&str> for various Arg::*_all() and
Arg::possible_values() methods this
commit now requires a generic IntoIterator<Item=AsRef<str>> which allows
things such as constant arrays. This change requires that any
Arg::*_all() methods be changed from vec!["val", "val"] -> let vals =
["val", "val"]; some_arg.possible_values(&vals) (or vals.iter()).
Closes#87