Support for cfg_attr (aka conditional derive)

[greyblake]

Addresses #188

This allows conditional derive of traits with cfg_attr(..). Works with derive and derive_unchecked.

Example

#[nutype(
    validate(not_empty),
    derive(Debug, Clone, PartialEq, Eq, AsRef),
    cfg_attr(feature = "serde", derive(Serialize, Deserialize)),
)]
pub struct Email(String);