Add "recover" method

[raveclassic]

Sometime we need to process RemoteFailure and return RemoteSuccess with some boolean flag, for example process RemoteFailure<AjaxError, boolean> 401 status and return const isAuthorized = success(false).

We should add a method called recover (may be discussed) accepting a callback transforming error to smth valueable. Similar to Filterable.filterMap such callback should return Some when transformation is applicable and None when not.
Type signature should be:
recover: RemoteData<L, A> ~> (f: (l: L) => Option<A>) => RemoteData<L, A>
Possible implementation for RemoteFailure<L, A>:

recover: (f: (l: L) => Option<A>) => {
  const result = f(this.error)
  return result.isSome() ? success<L, A>(result.value) : this
}

For RemoteInitial, RemotePending, RemoteSuccess should return self.