Conditionnal compilation

[Yamakaky]

It would be cool to support things like this (pseudo-code):

mod pote {
    error_chain! {
        errors {
            #[cfg(unix)]
            Unix {
            }
            #[cfg(not(unix))]
            Other {
            }
        }
    }
}

Currently, it works partially: the ErrorKind variant is generated correctly, but the matches always use the variants. Places which should be modified:

• impl Debug for ErrorKind
• impl Display for ErrorKind
• ErrorKind::description