Add Router.pathVar

[fdietze]

No description provided.

[cornerman]

I am hesitant to add alternative implementations for variables everywhere. What speaks against just converting the Router.pathSubject to a variable. It works like this: Var.combine(Rx.observableSync(subject), RxWriter.observer(subject)). Is that not enough?

[fdietze]

True. And in this case it's enough (maybe with a conversion helper for BehaviorSubject -> Var).

It doesn't work for the general case, though (as mentioned in #213 (comment)).

[cornerman]

We should a simpler converter from subject to var though - even if it is not correct for all cases.