text: Title: skopt.load/dump can not handle `Optimizer` instances
  Body: `Optimizer` instances can not be serialised with `skopt.dump/load`. This is probably unexpected for users, or at the very leat inconvenient. More discussion in #305
  
  It would make sense to extend `dump/load` to support "all" of the `skopt` classes etc.
  
  Needs a bit of discussing in terms of API as this is mainly syntactic sugar on top of `pickle`, so we need to find a way that is actually nicer to use than the bare `pickle`.