autoimpl trait re-implementation should copy cfg attrs

[dhardy]

Summary
The #[cfg(..)] attr is used to conditionally compile something. If a trait item is conditional, then its re-implementation should be too.

Are there any other attrs which should be forwarded?

To Reproduce

#[impl_tools::autoimpl(for<T: trait> &T)]
trait MyTrait {
    #[cfg(feature = "foo")]
    fn do_foo(&self);
}

[dhardy]

It seems that no other built-in attributes should be propegated to method implementations. (There is some ambiguity for allow, warn, deny, forbid since something like #[allow(non_snake_case)] can appear on both trait item declarations and implementations, but testing shows that this is unnecessary.)

[dhardy]

Fixed by #30