message: Make plugin feature optional
diff: #![feature(inclusive_range)]
  #![feature(drop_types_in_const)]
  #![feature(step_trait)]
  -#![feature(plugin)]
  +#![cfg_attr(feature = "clippy", feature(plugin))]
  #![cfg_attr(feature = "clippy", plugin(clippy))]
  #![cfg_attr(feature = "clippy", deny(clippy))]
  #![cfg_attr(feature = "clippy", deny(enum_glob_use))]
  