rx-stateful: change signature so that it is possible to pass argument from a trigger

[michaelbe812]

this should be possible:

nextPage$$ = new Subject<number>()
smth$ = rxStateful$((arg: number) => this.get(arg), {trigger$: nextPage$$})

tbd
eval if we should reuse refreshTrigger$ or add a new configuration property.
The latter will be more clear (probably) and also for future maintenance it will be easier