Make futures optional per dispatch call

[arximboldi]

#128 introduced a new feature to be able to track the completion of reduction of actions and effects. This introduces certain overhead, so it is made optional per store using the lager::with_futures reducer.

Ideally, however, this could be made optional per call to dispatch. There are two ways:

1. Add a different version of dispatch() (name suggestions? dispatch_future(), dispatch_(), dispatch_with(), ???) that returns a future, and have the default version not return one at all. In the current version, dispatch() always returns a future, but you get runtime errors when using the returned future if futures are not enabled in the store.

   This is probably cleanest option from an API POV but, in a trivial implementation, it imposes some penalty to creating and passing around contexes. Maybe there is a way we can instead of carrying around the dispatchers for various actions as function objects in the context, we create a static vtable for a particular combination of actions and parent context. Implementing this optimization, while a bit difficult, would also be more efficient than what we currently have for the general case.

2. Add a flag to dispatch(a, [flag]) to enable futures for each dispatch call.