variantly 0.2.0

Derive helper methods for enum variants that are familiar from `std::option::Option` & `std::result::Result` such as `unwrap_or` or `and_then`.
Documentation 
{
  "git": {
    "sha1": "e145c26e6f0088bc2df2d2c7d1dd34a693e07db4"
  }
}