Allow attributes on struct variant members

[canndrew]

This would be useful mainly for adding doc comments

quick_error! {
    /// We can put comments here
    pub enum MyError {
        /// And we can put comments here
        MyVariant {
            /// But we can't put em ere
            my_variant_member: MyType,
        } {}
    }
}

[tailhook]

@colin-kiegel, do you accept the challenge? :)

[colin-kiegel]

I'm quite busy right now, so I have to delay it right now.. So this might take a while.

[tailhook]

That's my situation too. Hopefully, this is not a big deal for errors...

[Yamakaky]

https://github.com/brson/error-chain/pull/55/files#diff-dbc69594b369f8b47dc0bf4b8af35b4f