Add callback option for subscribe methods

[quietlychris]

It's a common pattern in pub/sub libraries to support running an operation on subscribed values once they're received. An option should be available for the user to create a function that can be passed to a Subscriber Node to do this. It might take some consideration on how to manage this with the types generics in case the callback's return isn't the same as the initial requestee value (i.e. turning a usize into an f32 or something)