When used with `Arg::possible_values` it allows the argument value to pass validation even if
the case differs from that of the specified `possible_value`.
```rust
let m = App::new("pv")
.arg(Arg::with_name("option")
.long("--option")
.takes_value(true)
.possible_value("test123")
.case_insensitive(true))
.get_matches_from(vec![
"pv", "--option", "TeSt123",
]);
assert!(m.value_of("option").unwrap().eq_ignore_ascii_case("test123"));
```
This setting also works when multiple values can be defined:
```rust
let m = App::new("pv")
.arg(Arg::with_name("option")
.short("-o")
.long("--option")
.takes_value(true)
.possible_value("test123")
.possible_value("test321")
.multiple(true)
.case_insensitive(true))
.get_matches_from(vec![
"pv", "--option", "TeSt123", "teST123", "tESt321"
]);
let matched_vals = m.values_of("option").unwrap().collect::<Vec<_>>();
assert_eq!(&*matched_vals, &["TeSt123", "teST123", "tESt321"]);
```
Closes#1118
