Add custom name for Other variant

[slyons]

Addresses #446 and provides a way to specify the name of the fallback variant of the Query enum.

With serde_enum_fallback_variant_name=Some("Autre") you now get an enum like so:

pub enum AnEnum { 
  where_ , 
  self_ , 
  Autre (String) 
}

[tomhoule]

Hi @slyons, thanks for the PR! I am going to take on maintainership of this crate a bit more actively again, do you still want to take this PR to the finish line? It looks good, I think we should do it.