Change `subscriptionsFn` to take a model as an argument

[z0w0]

This is how Elm works, and I have to admit after working on an Elm project recently - it's 10x better then just returning subscriptions for the whole game state. It allows you to conditional define which subscriptions are needed at any point of runtime.

I think this is definitely possible with a few changes to how Elerea is inter-op'd with, I'm just struggling to realise what changes are required at the moment..