fork-join and zip should have a dedicated parameter for selector

I know this situation might not be general.

So I extended `beicon.core/Observable` to implement the `IFn` interface. But this led to problems in the arrangement of my arguments when using `zip`. Looking through the source code I discovered, that only fork-join and zip check if their first argument is a function before proceeding. I would like to suggest changing it to a variadic function(not sure if that's correct). Something like this

```clojure
(defn zip
([& items].....)
([selector & items...........]))
```