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

[noxecane]

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

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