Feature proposal: add reaction that allows the user to explicitly notify watchers

[grav]

This pull request is an extension to the api reagent.ratom/make-reaction-notify, that allows creation of reactions that allow for explicit notification of watchers.

This way, it is possible to create a reaction that for instance wraps asynchronicity:

(def input (r/atom 42))

(def squareroot-of-input (reagent.ratom/make-reaction-notify (fn [notify] (-> (js/fetch (str "http://heavymath.com/squareroot/" @input))
                                                      (.then notify)))))

(def result (reaction (* 2 @squareroot-of-input)))

(defn view []
   [:p (str "The double square-root of " @input " is " @result)])

Here, the squareroot-of-input reaction will fire when input changes, and then notify the result reaction when the xhr request is done.

It's possible to call notify several times in a function. It's not a new feature for FRP libraries (I've used it in RxJS and Reactive Cocoa), I just couldn't find anything similar in Reagent.

The implementation is very crude, so it's more a kind of proposal for a feature than a final implementation.