generate custom default values for struct fields
```rust use custom_default::CustomDefault;
struct Person {
#[default("Jack")]
name: &'static str,
#[default(12)]
age: u32,
#[default(true)]
dead: bool,
// without the `default` attr it will work like normal `#[derive(Default)]`
size: usize,
}
assert!(Person::default() == Person { name: "Jack", age: 12, dead: true, size: 0, }); ```