Add support for variant attributes

[philipp-haker]

I usually like to combine Kinded with other traits. Examples are:

• From<TraitEnumKind> for TraitEnum and Default for TraitEnumKind which is nice for enums containing implementors of a specific trait: let variant : TraitEnum = TraitEnumKind::default().into();
• another is EnumMessage from strum to display additional information about the kinds.

While the Default impl is easy to do in the first case, manually implementing EnumMessage is a tad more annoying.
As a solution I would like to propose that variant attributes can also be passed on like so:

#[kinded(derive(Default, EnumMessage))]
pub enum UserId {
  #[kinded(default)]
  #[kinded(strum(message = "User by ID", detailed_message = "Identify a user by their (unique) id"))]
  ById(u64),

  #[kinded(strum(message = "User by Username", detailed_message = "Identify a user by their username"))]
  ByName(String),

  #[kinded(strum(message = "User by email", detailed_message = "Identify a user by their email address"))]
  ByEmail(String),
}

This would also solve #18 like this:

#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(rename="camelCase")]
#[kinded(serde(rename="camelCase"))]
pub enum Event {
  #[serde(rename="LoggedIn"))]
  #[kinded(serde(rename="LoggedIn"))]
  Login(LoginDetail)
  Logout(LogoutDetail)
}

which does contain redundancies but is still better than the current way.

[greyblake]

Addressed in #27